1. Page Cache and Page Writeback
Chris Gill, Brian Kocoloski
CSE 422S - Operating Systems Organization
Washington University in St. Louis
St. Louis, MO 63130

2. System Architecture struct page * pages CPU DRAM System Bus
Peripheral Devices (hard drives, keyboards, adapters, etc.
Bridge
I/O buses
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3. Main memory (RAM) Characteristics Fast Limited Capacity
CPU
DRAM
System Bus
Peripheral Devices (hard drives, keyboards, adapters, etc.
Secondary memory (e.g., hard drive) Characteristics
Slow
Large Capacity
Persistent
Bridge
I/O buses
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4. Hardware Performance Differences
Performance comparison of most block devices with other hardware
Reading/writing a CPU register:
O(nanoseconds)
Access memory in CPU cache:
DRAM memory access:
O(microseconds)
Access to spinning hard drive:
O(milliseconds)
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5. CSE 422S – Operating Systems Organization
I/O Without Caching
Read/write block to/from I/O device
Without any caching, all reads/writes go to a (slow) storage device along a (relatively slow) I/O bus
Disk reads on the order of ms in time (~ 2 orders of magnitude slower than main memory)
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6. CSE 422S – Operating Systems Organization
I/O With Caching
Page
Read/write block to/from I/O device
Read block from storage
Allocate page of memory
Copy block contents into the page
Store page in the page cache
Memory write
Disk read
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7. CSE 422S – Operating Systems Organization
I/O With Caching
Page
On subsequent read/write of the same block
Is the block located somewhere in the page cache?
If yes, read/write directly from memory
Else, read/write from storage
Memory write
Disk read
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8. CSE 422S – Operating Systems Organization
Page Cache
Simple high level idea: save file contents in memory for faster future access
Q: does the overhead of checking the page cache matter?
Do we care about the overhead for when the cache misses?
What about when the cache hits?
Q: what other questions does this create?
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9. CSE 422S – Operating Systems Organization
Page Cache
Simple high level idea: save file contents in memory for faster future access
Q: does the overhead of checking the page cache matter?
Q: what other questions does this create?
How large should the cache be?
How to maintain consistency between memory and disk?
How frequently should cache/disk contents be synchronized?
What should happen on writes?
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10. Cache Size and Consistency
Size of the page cache grows/shrinks dynamically based on system workload
Only data that have been accessed are mapped
Page cache may hold different portions of different files
Write caching
Writes may bypass the cache entirely (so-called “no-write” approach)
Writes may update both page cache and disk (“write-through” approach)
Writes may update page cache and mark as “dirty” and then written back to disk later (“write-back” approach)
Allows requests to be merged and sorted, per previous lecture
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11. Write Caching Approaches
No-write
Only makes sense if you know the data will not be read/written again in the future
Write-through
Writes to the page cache are immediately written through to storage
Benefit: keeps cache coherent with physical storage
Downside: all writes to storage issued independently
Write-back
Writes to the page cache are NOT written to the backing store immediately
Downside: inconsistency between memory and storage contents (what happens if the system crashes?)
Benefit: multiple writes to backing store can be coalesced and performed in bulk, which the block layer can handle more efficiently (via I/O scheduling approaches)
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12. CSE 422S – Operating Systems Organization
Cache Eviction
Clean vs dirty
If not enough memory available, Linux replaces clean page(s) first
If no clean pages are available, Linux forces a writeback of a dirty page, and then replaces that page
Eviction
Clean page; no writeback needed
Dirty page; ”dirty” contents need to be written back to persistent storage, then the page can be released
Eviction Policy
Assuming all pages are dirty, what should be evicted?
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13. CSE 422S – Operating Systems Organization
Linux’ Cache Eviction
Cache eviction
LRU strategy: if we need to evict, find the least recently used clean page, and remove it from the cache; no writeback needed
Clean Pages
Dirty Pages
Sorted by LRU
Sorted by LRU
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14. CSE 422S – Operating Systems Organization
Linux’ Cache Eviction
Cache eviction
LRU strategy: if we need to evict, find the least recently used clean page, and remove it from the cache; no writeback needed
Clean Pages
Dirty Pages
Sorted by LRU
Sorted by LRU
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15. CSE 422S – Operating Systems Organization
Linux’ Cache Eviction
Cache eviction
If no clean pages are available, Linux forces a writeback of a dirty page, and frees the page or marks it clean
Clean Pages
Dirty Pages
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16. CSE 422S – Operating Systems Organization
Linux’ Cache Eviction
Cache eviction
If no clean pages are available, Linux forces a writeback of a dirty page, and frees the page or marks it clean
Clean Pages
Dirty Pages
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17. CSE 422S – Operating Systems Organization
Cache Eviction
Least recently used strategy (LRU)
Evicts page with the oldest access timestamp
Based on idea that “least recently used” is a good approximation for “least likely to be used in the near future”
An effective heuristic (Linux isn’t really clairvoyant)
Linux current strategy: Two-list strategy (LRU/2)
Addresses issue where file contents are only ever read once
Linux modifies LRU slightly by keeping two separate lists
Pages move from inactive list to active one when they are accessed
Within each list, pages are ordered by access timestamp
If active list is larger than inactive one, older pages move to inactive
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18. Linux’ Cache Eviction Two-List LRU
Active List (clean, but cannot be evicted)
Inactive List (clean, and can be evicted)
Considered “hot”
Considered “cold”
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19. Linux’ Cache Eviction Clean Pages Page accessed
Active List (clean, but cannot be evicted)
Inactive List (clean, and can be evicted)
Page accessed
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20. Linux’ Cache Eviction Clean Pages Page accessed (moves to active list)
Active List (clean, but cannot be evicted)
Inactive List (clean, and can be evicted)
Page accessed (moves to active list)
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21. Reading file contents into memory
Page
Memory write
Q: What is the Linux system call we’ve used that performs very similar operations?
Disk read
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