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1 Caches Electronic Computers LM Parallelism. 2 Cache For sake of simplicity let’s suppose there is only one level of cache The cache is a memory with.

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Presentation on theme: "1 Caches Electronic Computers LM Parallelism. 2 Cache For sake of simplicity let’s suppose there is only one level of cache The cache is a memory with."— Presentation transcript:

1 1 Caches Electronic Computers LM Parallelism

2 2 Cache For sake of simplicity let’s suppose there is only one level of cache The cache is a memory with an access time some order of magnitudes shorter than that of che main memory BUT with a size much smaller. It contains a small (see later) replicated portion of the main memory. The CPU, when accessing a data, tries FIRST to find it in cache (hit) and then, when the data is not found, in the main memory (miss) In cache there are no single bytes BUT groups of bytes with contiguous addresses (normally 32 o 64 o 128 and in any case “aligned”): each group is called line LOCALITY PRINCIPLE (SPATIAL AND TEMPORAL) WORKING SET CPU Registers Cache I lev. Cache II lev. Cache III l3v. Memory Disk Tape Cache Parallelism

3 Cache 3 0 Memory 32-256 bytes per line 0 Cache 2 5 m m+1 n n+2 2 5 m m+1 n n+2 Memory access time >100 clock cycles Cache access time : 1 to 4 clock cycles Line numberIn line offset Processor generated address Cache position detection Cache line Data Data line Number of line: the address of the lower byte of a line divided by the size of the line (aligned). In other words the line numer is the complete address of the first line byte minus the LSbits which are zeros (alignment!) Data line address Accessed data range: single byte to the entire line Parallelism

4 4 Memorie associative (Content Addressable Memories) Associative memories : they include BOTH data lines and the of the lower byte address (line number - TAG) A data is found not through the decoding of the CPU address BUT by mean of a parallel comparison between all cache lines numbers (TAGs) and the CPU MSB address. The comparison can be either successfull (hit) or not (miss) Line number Data Line number A line size can be one byte only (never used)! Parallelism

5 5 Full-associative cache 315 TAG 0 Slot 1 Validity 7225 1 0 7226 2 1 57 m 1 88 n 1 Line Cache Line 0 Line 1 Line 2 Line k Line w Line w+1 Line z Memory In each slot any memory line can be stored. The TAG is the line number For instance: 64GB memory (36 bit address) and 256 byte lines. Offset in line: 8 bit. Tag=36-8= 28 bit 256 bytes/line The line number is compared with all cache TAGs. In case of HIT (and if the validity bit is 1) the requested data is present. The address offset is the position of the first byte in the line (requested data can be a byte, a word, a double word and so on. provided it is within the line boundary). This cache organization makes the best use of the cache but it is terribly complex since it requires many comparators (if the cache has 1024 slots - in this case the cache size is 256 Kbytes - 1024 28 bit comparators are required!) and normally caches have 64K slots and more Cache size is always a power of 2 as the line size Line Number Line number (28 bit) In line offset (8 bit) Processor generated address Parallelism

6 6 Directly mapped cache TAG 0 Slot 1 Validity 1 0 2 1 m 1 n 1 Line Lina Line Cache In each cache slot only a subset of the memory lines can be stored. For instance in slot 0 only those whose initial address is exactly the divison of line number by the slots number, in slot 1 only those whose initial address is exactly the division of the line number by the slots number with remainder 1 and so on. Obviously the initial memory address of data in each slot is the line number mulpiplied by the line size For instance: 1 MB memory, 64 bytes lines, 16K different lines, 16 line numbers. If the cache has 128 slots => 16K divided by 128 is 128. Therefore in slot 0 lines number 0, 128, 256, etc., in slot 1 lines number 1, 129, 257 etc. Line 0 Line 1 Line 2 Line k Line w Line w+1 Line z Memory

7 Parallelism7 Line 0 Line 1 Line 2 Memory Line 3 Line 4 Line 5 Line 6 Line 7 Line 8 Line 9 Line 10 Line 11 Line 12 Line 13 Line 14 Line 15 TAG 0 Slot 1 Validity 1 0 2 1 Line Cache 3 1Line Cache directly mapped An example (line 4 bytes)

8 8 Directly mapped cache The LSBs of the line number indicate the only cache slot where the line can be stored. See the previous example of a processor with 36 bit address (64 GB), 256 byte line (8 bit): the line number is 28 bit (how many lines ? 2 28 -> 2 10 x 2 10 x 2 8 ). If the cache has 1024 slots (256KB) the 10 LSBs of the line number (index) indicate the slot where a line must be stored In line offset (8 bit) TAG (18 bit) Slot (10bit) Only one 10 bit decoder (to detect the involved slot) and only one 18 bit comparator are needed Very little flexible Index Line number (28 bit) In line offset (8 bit) Processor generated address Parallelism

9 Directly mapped cache 9 TAGDATA Offset in line TAG Slot Cache In each slot only one line for each index can be stored Index Processor generated address Parallelism

10 Cache 10 A compromise n-way set-associative cache N-way set-associative : many lines for each index N comparators for n-way. Parallelism of the comparators identical to that of directly mapped cache In the directly mapped caches data can be provided before validity and TAG check. In the set-associative caches only after the check Sometimes speculative mechanisms (way 0 data is provided then check) TAGDATA Offset nel blocco TAG Slot Processor generated address Parallelism

11 11 Cache set associative INDIRIZZOBLOCCO DI CACHE TagIndexOffsetStatusTagDato Data word Hit/miss TAG check and data selection according to the data type requested by the CPU (byte, word, DW etc.) Way 0 Way 1Way n Way 0 Way 1Way n Way 0 Way 1Way n Way 0 Way 1Way n Parallelism

12 Therefore... 12 In a fully associative cache a line can be stored in any slot In a directly mapped cache in only one slot, that corresponding to the INDEX In a set-associative cache in any way of the slot corresponding to the INDEX efault.htm age3.htm rame2.htm Parallelism

13 13 Replacement algorithms Caches are of limited size and therefore is necessary (i.e. in case of a read miss) to select a line which must be discarded (overwritten if not modified, written back in memory and then overwritten if modified) Ther are basically three possible policies: RAND (Random), LRU (Least Recently Used), and FIFO (First In First Out) with different efficiency and complexity RAND: in this case the logical network must first detect whether invalid lines are present (and therefore overwrite one on them): if not according to a random number generator (i.e. a shift register feedbacked by an EX-OR gate) must select a line to be replaced. The algorithms can be refined selecting first the non-modified lines. Although non-optimal this algorithm is very cost-effective Parallelism

14 14 Replacement algorithms STACK: the same network for each set. When a“hit”occurs the hit way must become the most recent and all others become of a lower rank with no change among them. Let’s suppose there are 4 ways and that all lines of the set are valid. The way (its number) in position Ra is the most recenttly hit. The other lines were hit in the past according to their positions. Na, Nb, Nc, Nd store the four way number 0,1,2,3 (obviously not in order!! It depends on the set history! ) Rx as Ra, Rb,Rc,Rd: 2 bit registers X: hit way number (if any) Rd stores the way number least recently hit. Its line is the candidate for replacement in case of miss for the set: the way where the read dat are stored after the replacement becomes the most recently hit and its number is stored in Ra while all other way number are right shifted one position NaNbNcNd AND CLK X Ex-OR RaRbRcRd Parallelism

15 15 Replacement algorithms Let’s now suppose a HIT for way 2 and that the way numbers in R i registers from left to right are 1, 0, 2, e 3. (Way 3 is the replacement candidate in case of set miss). The shift register right shifts until Rc (whose way number is 2) because Rd clock is blocked. After the clock the R i registers store (in sequence) 2, 1, 0, 3 (way 3 is still the candidate for replacement while all other way numbers are correctly updated with way 2 as the most recently hit) When a line is invalidated its way number is stored in Rd and all other ways numbers which were hit less recently than the invalidated line are left shifted one position The mechanism is symmetrical to the hit mechanism. For instance that in presence of the situation depicted in figure line 0 (in register Rb) is invalidated. Line 0 is stored in Rd while line 2 is stored in Rb and line 3 in Rc. In order to deal with the invalidation a symmetrical circuit must be added. 1023 AND CLK X Ex-OR RaRbRcRd 2 Parallelism

16 16 COUNTERS: a counter for each way of each set The counter walues correspond to the way ranking position for replacement: 0-> most recently hit, 3-> least recently hit Eventi 01) Hit Way 0 02) Miss (line fill – Way 2 count 3 replaced 03) Way 1 invalidated 04) Hit Way 0 05) Way 3 invalidated 06) Miss (line fill – Way 3 count3 replaced) 07) Hit Way 2 08) Miss (line fill – Way 2 count 3 replaced) 09) Miss (line fill – Way 0 count 3 replaced) 10) Miss (line fill – Way 0 count 3 replaced) Validità 1 1 1 0 1 1 1 0 1 0 1 1 1 1 In most implementation the counters can be incremented or reset. In case of hit of a way number the counters with a lower value are incremented and is reset the counter corresponding to the hit way. In case of miss and replacement the way whose counter is three is selected and then the system behaves as if that way was hit. In case of invalidation the invalidated way counter becomes 3 and all other counters with a greater number are decremented W0 W1 W2 W3 0 1 3 2 1 2 0 3 1 3 0 2 0 3 1 2 0 2 1 3 1 3 2 0 2 3 0 1 3 0 1 2 0 1 2 3 1 2 3 0 Finale status W0 W1 W2 W3 1 0 3 2 0 1 3 2 1 2 0 3 1 3 0 2 0 3 1 2 0 2 1 3 1 3 2 0 2 3 0 1 3 0 1 2 0 1 2 3 Initial status Replacement algorithms It must be noticed that the counter algorithm is equivalent to the shift register network. In that the position indicates the age rank, in this the counters Parallelism

17 17 Replacement algorithms PSEUDO-LRU (in this example 4 ways) The 4 set ways are indicated by I0, I1, I2 e I3 When a line is invalid it is replaced in case of miss There are three bits (B0, B1 e B2) each set If the last set access was for I0 or I1 then B0 =1 otherwise B0=0 If the last access for the two ways I0 and I1 was for I0 then B1=1 otherwise B1=0. If the last access for the two ways I2 and I3 was for I0 then B2=1 otherwise B2=0 In case of replacement According to B0 the cache selects first which couple (I0:I1 or I2:I3) was least recently accessed then selects within the couple the way to be replaced according to B1 or B2 B0=0 ? Yes (I0:I1) least recently accessed B1=0 ?B2=0 ? YesNoYesNo I2I3 I0 I1 Replace The algorithm is pseudo-optimal because I1 could be the way least recently accessed but could be «blackened» by I0 if this is the most reently accessed. No (I2:I3) least recently accessed Parallelism

18 18 Replacement algorithms FIFO In this implementation there is a single counter for each set which starting from 0 is incremented for each read miss (that is for each replacement). The new line id inserted in the way pointed by the counter. This algorithms has a singularity because it does not consider the invalidations. If the counter has value 3 and line in way 2 is invalidated, way 2 and not 3 should be used in case of read miss. Although suboptimal this algorithm has a very good cost/effectiveness ratio. ame1.htm e/ Parallelism

19 What is then a TLB? 19 Processor Cache RAM Miss Virtual Address Physical Address Hit TLB Dati The TLB is a cache which instead of providing memory data provides memory addresses (physical addresses) since it is addressed by processor virtual addresses The TLB access time is similar to that of the 1st level cache. In the modern processors the TLB (like the caches) has two levels NB: the processors (theoretically today) could be not paged. In this case the TLB does not exist since the virtual addresses are also the physical addresses As for the the caches the TLB can be fully associative, directly mapped or set associative with the same replacement problems. TLBs are normally 8-16 ways set associative 64-1024 slots x.html Parallelism

20 Virtually addressed caches 20 Processor Cache RAM Miss Virtual Address Hit Data In this case an indirect address level (TLB or in the worst case the page tables) is spared but: Different virtual addresses can be mapped at the same physical addresses (for different processs) and therefore a process tag must be inserted in the cache and a possible duplication of the same data could take place. In case of RAM or cache line data change all slots containing the line must be invalidated which implies a complex hardware. Otherwise all data cache must be flushed upon a context switch. Parallelism

21 21 Cache write Two possible policies: Yes : write-allocate No: no-write-allocate N.B.: Write operation are VERY less frequent than the read operations and with a high probability of sparse addresses. How lines are dealt with in case of write miss ? Read (with possible replacement) and then write ? In case of write-allocate the operation is a read/replacement followed by a line write in cache. In the other case data are written on the following cache level (if any and containing the line, otherwise in memory) Parallelism

22 Cache write 22 When a write hit occurs ? Data must be written also in the following cache levels? Two policies Yes : write through No : write back In the first case the line is overwritten and data are also written in the following cache levels (down to the memory). In the second case a line is overwritten without forwarding the data to the next level cache (unless for coherency problems – see later). When a line must be replaced an already overwritten line must be first written back in the following cache level since data in the first level are more recent. The data traffic is much smaller (smaller bandwith use) but hadware is more complex It must be underlined that a line is a consistent data structure and therefore even in case of a single byte modification the entire line must be written back. All modern processors use the write back policy with the “write once” system which will be explained later Write-back policy implies that a bit for each line must be present in order to indicate whether the line has been modified (dirty bit) Parallelism

23 Posted write 23 Very often in order to reduce the access time impact the posted-write methodology is used Processor Cache FIFO RAM Data to be written in RAM are inserted in a FIFO write buffer which is accessed by the processor (or by the cache in case of write back for replacement) with no delay. The memory controller transfers then data from the buffer to the memory at the memory speed (much lower) Normally the FIFO slots are 4-32. When the FIFO is full, processor (or cache) is delayed. NB When the write buffer is used the cache read system must first check whether the requested data are in the FIFO Parallelism

24 Coherency 24 Caches have coherency problems This means that the system must grant the most recent data to a system «agent» (processor, DMA, graphic processor..) upon a read request. The coherency problem arises not only between caches of processors belonging to a multiprocessor system but also between different levels caches of the same processor. For sake of simplicity let’s consider that all processors of the same multiprocessor system have two levels caches (L1 and L2) and the common memory. In most cases L2 is bigger than L1 (the cache directly connected to the processor). Let’s suppose the caches are inclusive that is if a line is present in L1 it is present also in L2 (but not viceversa). The presented mechanism can be easily extended to the case of n-levels caches Parallelism

25 25 Coeherency policies READ How can we grant that an external agent (not the processor) reads from memory the most recent version of data (the data in memory could be stale that is «old») ? Let’ consider the write policies Write-through For each processor data write (both data present or not present in cache) the data is written also in memory: the coherency is therefore granted but the system is slowed by the memory access time. Posted write-through Similar to the previous case. The processor efficiency is improved (the processor is not normally delayed by the memory access time). No access is allowed to the external agent until data are written to the memory (not easy to implement and little efficient) ) Write-back In this case the memory is updated only when necessary (i.e. a replacement). For each external agent access the cache (or the caches) mut be checked in order to verify whether it (they) stores the requested data and if the aswer is positive the agent memory access must be blocked until the requested data are forcedly written back to the memory. Cache snoop mechanism Parallelism

26 26 Coeherency policies WRITE What happens when another agent wants to write data in memory ? Write-through The cache controller must monitor the system bus and invalidates in cache the lines (if any) containing the data overwritten in memory by the agent (until then coherent) Write-back The cache controller must monitor the system bus, and in case of an agent attempt to write must perform the following operations: a)If data are present in cache in a modified state line (or lines) the controller must stop the agent memory access, must trigger a write-back of the modified line and then invalidates the line (lines). It must be noticed that the write-back operation is needed because since a line is made of several bytes there is no way of detecting which byte (or bytes) were modified. The new master could write bytes different from those which were modified b)If line data are not modified upon a write from another master the line must be only invalidated in cache. Parallelism

27 27 Two levels caches coherency policies L1 e L2 write-through For each processor write (both in case data is present in cache or absent) data are written down to the memory. This obviously has a great impact on the bus, the most important bottleneck. The write operation by L2 could be deferred. In case of write of another agent data are invalidated (if present) in both cache levels L1 write-through e L2 write-back In this case L2 must monitor the bus and when another agent tries to read a data must first write back the modified data (if any – data are in any case the same in L2 and in L1 – if in L1 are present) in memory. In case of write acces by another agent, modified data must be first written back to memory then invalidated both caches. N.B. The processor has no way of determining whether a secondary cache is present. Signals exchanged with the system must be the same whether a secondary caches exists or not. The same applies for the secondary cache if a third level caches is present. How can we grant that another agent reads from memory the most updated data (if the same data were also in cache, the corresponding data in memory could be «stale» that is «older» than those in cache) ? Parallelism

28 28 Two levels caches coherency policies L1 and L2 both write-back When the processor reads data (line fill) upon a miss in L1, L2 checks whether it stores the requested data. If yes data are transferred to L1 (with a possible replacement). If the data are present in L2 this means that they are «cacheable». If data are non available in L2, data are requested to the memory controller (MC). If data are «cacheable» a line fill takes place both in L2 and L1. If not, data are simply read by the processor. In case of a processor write operation with both L1 and L2 write- back there are many cases which depend whether the system is mono- or multi-processor : in any case the system must provide the most update data when they are requested MESI PROTOCOL Parallelism

29 29 M.E.S.I. (monoprocessor - write back) M – modified (L1 and L2) The requested line is available in cache where it was modified without write-back downstream (which is L2 for L1 and memory for L2). The considered cache stores updated data. Notice that if a line is in modified state in L1 and L2 the line in L1 is more updated than the same line of L2. A write operation triggers a transition from M to M state without downstream write E – exclusive (L1 and L2) The considered line is present and identical to the same line present in the device downstream (which is L2 for L1 and memory for L2). A write operation triggers a state change from E to M without downstream write. (Careful: the name can be misleading) S – shared (state possible only for L1 in a monoprocessor system) The line is present il L1 (S), L2 (E) and memory. A write operation triggers a a downstream write upon which L1 state becomes E and L2 state is changed from E to M (no memory write.. see state E). L2 in mono processor systems is never in shared state because there are no agents which need to be informed of the state of the (single) processor internal line (which is not the case of multiprocessor systems) I – invalid (L1 and L2) The requested line is not available in cache N.B. Lines of a code cache can be only in S or I state At the system start-up alle lines in all caches are invalid Parallelism

30 30 Possible States of the same line L2L1 II M E M Not present E S Not present NB: Not present: line not present because we consider inclusive caches L2 never shared in mono processor systems !!!L1 is always in a state which is related to the state of L2. A line cannot be in M- state in L1 if not in M –state in L2 Monoprocessor case (with two levels caches) Parallelism

31 31 Coherency policies (L1 and L2 both write back) In case of monoprocessor systems a line-fill when data are not present both in L1 and L2. L1 state becomes S and L2 state E. A successive write operation to L1 triggers a state change of L1 to E and L2 to M (the L1 written data are also written to L2). Data are not writte to memory A successive write operation affects only L1 whose state becomes M. NB: Since the size of L2 is bigger than the size of L1 it is possible (because of replacements) that a line is not present in L1 but in L2 only either in E or M state. A line fill, therefore in L1 stores the line in L1 respectively in S or E state. In the following slides we assume that all caches are inclusive. The MESI protocol is however applicable also to other cases Parallelism

32 32 Coherency policies (monoprocessor) Read operation. If L2 line containing the requested data is in E- state then the same line is in S-state in L1 (if any). Memory data are therefore the most updated. Write operation It triggers an “enquiry” of the Memory Controller in L2. If the line (if any) containing the data is present in L2 in E- state then the same line is in S-state in L1 (if any). The line in L1 and L2 is invalidated. If the line (if any) in L2 is in M-state, L2 must check in L1 whether the line is present in L1 and is in M-state. In any case the most recently updated version of the line is written back to the memory and the line is invalidated in both caches. The the external agent can then write its data in memory. The following cases apply to external agents without private cache (i.e. DMA controller) accessing memory Parallelism

33 33 PROCESSOR READ COERENCY No external caches Monoprocessor 1) Miss in L1 e not in L2. Line fill from L2 a L1. L1 state depends on L2-state. If L2 state is exclusive, L1 becomes shared; if L2 state is modified L1 becomes exclusive. No chance of a line present in L1 and not in L2 2) Miss in L1 e L2 -> double line fill. L1 > shared and L2 -> exclusive N.B. Why must L1-> S if L2 is exclusive ? Because in case of write if L1 were in exclusive state no write-back to L2 would take place (l1 E->M) and a memory enquiry would find that the requested data in L2 are identical to those in memory (although stale) and no further enquiry on L1 would take place. A read or write data of an external agent would operate on the memory data without write-back of L1 data (the most recent data) Parallelism

34 34 PROCESSOR WRITE COERENCY No external caches Monoprocessor 1)Miss in L1 and L2: line fill from memory in L2 (->E) and L1 (->S) then write to both caches (L1-> E and L2->M) 2)Miss in L1 and not in L2. Line fill from L2 in L1 then write. If L2 in E state L1->S otherwise L1-E (L2 can only be in E or M state). Final states as per point 1 3)L1 hit. Tre cases (the line is surely in L2 too) a) L1 shared (and therefore L2 exclusive). Write to L1 and L2. L2->M and L>-E. b) L1 exclusive (and therefore necessarily L2 modified). Write to L1 only. L1->M c) L1 modified (and therefore L2 modified): write to L1 only. L1 remains in M-state Parallelism

35 35 External agent READ/WRITE coherency Cacheless external agent External agent READ 1)Miss in L1 and L2 or HIT in LI or L2 both not modified: NOP 2)Hit in L1 modified (and therefore L2 modified): L1 write back to memory and L2. L1->S and L2->E 3)Hit in L2 modified (e L1 exclusive or line not present in L1): L2 write back to memory. L2->E and L1 (if any) ->S External agent WRITE 1)Miss in L1 and L2: NOP 2)Hit in L2 and possibly in L1 both not modified: L2->I and L1 ((if any) ->I 3)Hit both in L2 and L1 (both modified): write back to memory of L1 then L1->I and L2->I Parallelism

36 36 M.E.S.I. (multiprocessor) M – modified The line is present only in the caches of one processor and in the specified cache it was modified without being written back to the downstream device (is is different form the same line in the downstream device). The line can be read and written without any downstream cycle. E – exclusive The line is present only in the caches of one processor and its content is identical to the downstream device. The line can be read and written without any downstream cycle. A processor write operation provokes a transition to M state. S – shared The line is possibly in the caches of many processors. (Possibly because it could be present, for instance, in two processors and then one of them has replaced the line) A write operation causes a downstream write and invalidates the line in the caches of other processors, if any. I – invalid The requested line is not available in cache. A read operation causes a LINE-FILL. A write operation causes a WRITE-THROUGH in case of non write-allocate policy otherwise a line fill followed by the write operation Parallelism

37 37 Possibile States of the same line Multiprocessor case (with two levels caches) L2L1 II M E M Not present E S Not present S S Not present In case of multilevel caches a lower level cache stores a reduced set of the lines of the upper level (inclusive caches). But not always (not inclusive caches) ! Parallelism

38 38 READ COHERENCY Multiprocessor (only L1 and L2) 1)Miss in L1 but not in L2. When L2 shared or exclusive L1 the read line becomes shared When L2 modified, L1 the read line becomes exclusive. NB: Similar to monoprocessor case but notice that in this case is it possible that both L1 and L2 are or shared (while in case of a monoprocessor L2 IS NEVER in S state) Parallelism

39 39 READ COHERENCY Multiprocessor 2) Miss in L1 and L2. Bus snoop When the line in neither cache a double line fill occurs. If not present in caches of another processor in L1 the read line is in shared state and in L2 is in exclusive state When the line is present in some other caches not modified (that is is in shared or exclusive state) upon the snoop all become shared state, The line is read into L1 and L2: in both caches of the requesting processor (as in alla caches of the orher processors) the state become shared If the line is present in the caches of only one processor and is in modified state (a line can be in modified state ONLY in one processor !) back-off on the bus, write back of the line in memory, the hit caches state becomes shared. The line is read into L1 and L2: in both caches the state become shared Notice that if a line is in modified state in a L1 is in modified state in the corresponding L2 too !! N.B. A bus snoop is a snoop on L2 which is forwarded to L1 if L2 is in modified state Parallelism

40 40 WRITE COHERENCY Multiprocessor 1)Miss in L1 e L2..Three cases a)The line is not in caches of other processors: as for the monoprocessor b)The line is present in other caches not modified: all caches containing the line are invalidated. Read in L1 and L2 and then write; final state L1 exclusive and L2 modified c)The line is present in another processor (only one !) in modified state. Bus back-off, the modified line is written back to the memory and the caches storing the line invalidated (do not forget that both L1 and L2 can be in modified state). The modified line must be first written back because it is not known which data of the line will be rewritten. Then as in the case of the monoprocessor. In any case at the end of the operation L2 modified and L1 exclusive. 2)Miss in L1 and not in L2. The line stored in L2 is forwarded to L1 and then written. Three cases a)L2 exclusive. No bus snoop, The line is written in L2 and L1. At the end L2 modified and L1 exclusive. b)L2 modified. L2 modified and L1 modified c)L2 shared. Bus snoop with invalidation, read in L1 and L2 and then write operation. L1 exclusive and L2 modified Parallelism

41 41 WRITE COHERENCY 3) Hit in L1 (and therefore in L2). Three cases: a)L1 modified. Only L1 is written b)L1 exclusive. Only L1 is written. L1 modified c)L1 shared. Two cases : I.L2 is shared. Bus snoop with invalidation then write on L1 and L2. L1 exclusive and L2 modified I.L2 exclusive. No bus snoop- Write on L1 and L2. L1 exclusive and L2 modified N.B. There are no cases with L1 shared and L2 modified, Parallelism

42 42 Other coherency policies “Directory based” coherency protocol The total memory is the sum of the processors local memories (accessible also from other processors) and the common memory- There is therefore an unique memory addressing system for all memories Information about memory lines are stored in a directory associated to the block Each directory stores the information about the line and the processors whose caches (if any) store the line Each line can be in the following states Shared: one or more processors caches have the line coeherent with the memory Non cached: no processor caches has the line Modificata: Only one processor cache has the modificed line. In this case the processor is the owner of the line (Common memory) MC M P1 C1 I/O D M P2 C2 I/O D M P3 C3 I/O D M P4 C4 I/O D D Directory Caches (possibly multilivel) Parallelism

43 43 Directory based protocol In the line directory there is a bit for each processor which is 1 if the processor cache stores the line. Two or more 1’s mean that the line is in shared state. A single 1 means that there is a possible owner of the line (the line could be in modified state). If a line is modified in a cache a message is sent to the directory which send a message to invalidate the other caches. In case of read a message is sent to the owner (if any) which must write back ita modified data and the line become shared. In case of write write-back and invalidation of the previous owner (if any).. The transitions are simililare to those of MESI but the implementation is different. This system is very useful if there are multiple connections between the processors by reducing the global use of the busses. MC M P C I/O D M P C D M P C D M P C D D L L L Home Remote Local request message L=line Parallelism

44 Caches Parallelism44 There are two types of caches: unififed and not-unified. Not-unified means that data and instructions are not mixed. Unified menas the contrary In general in the modern processors the first level caches are not-unified (Harward architecture). Other levels are unified

45 45 R2000/R3000 - RISC (DLX reale) IF/IDID/EXEX/MEMMEM/WB IFIDEXMEMWB I Cache D Cache Memory Harvard architecture Pipeline split-cycle (Phases  and  IF ID EX MEM WB  Virtual address translation TLB  I-Cache access - If hit  instruction read and parity check  Registers read – if Branch destination address computing  Start ALU execution – if Branch check condition  End ALU – if Load/Store vitual address translation (TLB)  D-cache access if write) (  Data from D-Cache if read  Register File write  Norally the branch condition is tested at the end of EX (two other instructions already started). In this case the test occurs in  and the I-cache addressing in  and therefore only one instruction penalty (the instruction in ID stage) 11 11 11 11 11 Feedback Branch Dati Physical addresses Parallelism

46 Web site Parallelism46 At this address some interesting animated views of the caches behaviours

47 47 Branch Target Buffer In order to avoid stalls derived from branches, a branch prediction is necessary in the first stage of the pipeline. The prediciton can be either correct or wrong. In any case the branch is tested in the execution stage. P C PC address Destination address T/UPC address Destinatino address T/UPC address Destination address T/UPC address Destination address T/UPC address Destination address T/UPC address Destination address T/U The BTB is a cache whose TAGs are the addresses detected as branches. The line in this case is the branch destination address and among the status bits there are those who predict whether the brach is taken or Untaken In case of miss (detected in the execution stage) a line fille occurs and a replacement procedure is activated. The initial prediction is that occurred in the execution stage Branch Target Buffer Taken Untaken Parallelism

48 Branch prediction 48 How is a prediction managed ? On a statistical basis ? Simple case: static prediction. The prediction is always «taken» The error probability with this policy, according to SPEC benchmarks, is 34% (fairly high) Static prediction according to the direction of branch (forward or backward) In this case the prediction is taken for backward branches (see loops) and the prediction is untaken for forward branches In SPEC benchmarks, however, the majority of branches id forward and the prediction is taken, therefore the prediction gives better results Dynamic prediction on the basis of the history of the branch The prediction error varies between 5 to 22% Parallelism

49 49 Branch Target Buffer With only one prediction bit which records the last verified branch. In this case for loop1 there are two successive prediction errors Loop1 Loop2 When loop2 ends (predicted as taken but untaken) there is a following error because in the first following loop loop1 will be predicted as untaken Parallelism

50 50 Branch Target Buffer Normally two bits are used. Two possible schemes TAKEN UNTAKEN TAKEN UNTAKEN TAKEN UNTAKEN TAKEN UNTAKEN TAKEN UNTAKEN TAKEN UNTAKEN TAKEN UNTAKEN TAKEN UNTAKEN In this case after two «mispredictions» the prediction is changed (low pass filter) In this case after two «mispredictions» the prediction is changed but ready to go back to the previous prediction in case of a further change With both schemes the accuracy is higher than 80% Parallelism

51 Simulator Parallelism51

52 Advanced algorithms for BTB 52 Two levels adaptive prediction Two registers: BHR (Branch History Table) and PHT (Pattern History Table) First case: globale approach 00101110 Ex: BHR (Shift Register) (content = 2E h ) 00101 01100 11100 10101 (00) (01) (2E) (FF) PHT 1 -> Branch taken 0 -> Branch not taken History of the most recent n (8 in this example) branches (what really happened, that is whether the the branch was verified as either taken or untaken What was predicted with the same global succession (BHR) ? Decision: taken Decisione: untaken In this example the content of the BHR is 2E h =47 10 Parallelism

53 53 Advanced algorithms for BTB In case of branch the most recent event succession is analysed (whether the branch was really taken or untaken), For each configuration of this succession a pattern is selected which reflects the decisions taken with this succession configuration. After each Branch execution the resulted value is stored in the right-shifted BHR A function must be defined which according to the contents of the BHR and the PHT predicts the branch This prediction system (which uses n + (2**n x m) FF - where n is the size of the BHR and m that of each PHT slot) is not particularly significant because there is no difference among all branches. Effective but not very precise. Parallelism

54 54 Advanced algorithms for BTB Second case: mixed preditor In this case there is a BHR for each branch made of K shift registers each one of n bits (one for each branch) while there only one PHT. m K n 2**n K branches considered Branch (address) BHT Same pointed PHT Parallelism

55 55 Advanced algorithms for BTB N.B.: registers related to different branches can point to the same PHT register In this case too there is a lack of consistency: while the history of each branch is different the originating pattern is the same Used FFs: k x n + (2**n x m) Parallelism

56 56 Advanced algorithms for BTB Third case: omogeneous predicto r A (complex) refinement of the second case m Required FFs: k x n + (2**n x m x k) k n 2**n k Parallelism Branch (address) BHT

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