On 10/24/2023 9:48 PM, Kefeng Wang wrote:
On 2023/10/24 20:56, Baolin Wang wrote:
Now ptep_clear_flush_young() is only called by folio_referenced() to
check if the folio was referenced, and now it will call a tlb flush on
ARM64 architecture. However the tlb flush can be expensive on ARM64
servers, especially for the systems with a large CPU numbers.
Similar to the x86 architecture, below comments also apply equally to
ARM64 architecture. So we can drop the tlb flush operation in
ptep_clear_flush_young() on ARM64 architecture to improve the
performance.
"
/* Clearing the accessed bit without a TLB flush
* doesn't cause data corruption. [ It could cause incorrect
* page aging and the (mistaken) reclaim of hot pages, but the
* chance of that should be relatively low. ]
*
* So as a performance optimization don't flush the TLB when
* clearing the accessed bit, it will eventually be flushed by
* a context switch or a VM operation anyway. [ In the rare
* event of it not getting flushed for a long time the delay
* shouldn't really matter because there's no real memory
* pressure for swapout to react to. ]
*/
"
Running the thpscale to show some obvious improvements for compaction
latency with this patch:
base patched
Amean fault-both-1 1093.19 ( 0.00%) 1084.57 * 0.79%*
Amean fault-both-3 2566.22 ( 0.00%) 2228.45 * 13.16%*
Amean fault-both-5 3591.22 ( 0.00%) 3146.73 * 12.38%*
Amean fault-both-7 4157.26 ( 0.00%) 4113.67 * 1.05%*
Amean fault-both-12 6184.79 ( 0.00%) 5218.70 * 15.62%*
Amean fault-both-18 9103.70 ( 0.00%) 7739.71 * 14.98%*
Amean fault-both-24 12341.73 ( 0.00%) 10684.23 * 13.43%*
Amean fault-both-30 15519.00 ( 0.00%) 13695.14 * 11.75%*
Amean fault-both-32 16189.15 ( 0.00%) 14365.73 * 11.26%*
base patched
Duration User 167.78 161.03
Duration System 1836.66 1673.01
Duration Elapsed 2074.58 2059.75
Barry Song submitted a similar patch [1] before, that replaces the
ptep_clear_flush_young_notify() with ptep_clear_young_notify() in
folio_referenced_one(). However, I'm not sure if removing the tlb flush
operation is applicable to every architecture in kernel, so dropping
the tlb flush for ARM64 seems a sensible change.
At least x86/s390/riscv/powerpc already do it, also I think we could
Right.
change pmdp_clear_flush_young_notify() too, since it is same with
ptep_clear_flush_young_notify(),
Perhaps yes, but I'm still unsure if removing tlb flush for PMD entry is
applicable to all architectures. Let's see the discussion in this
thread. Thanks.
