Re: [PATCH v10 00/12] LUF(Lazy Unmap Flush) reducing tlb numbers over 90%

Byungchul Park
2024-05-22T10:27:56Z

On Wed, May 22, 2024 at 03:38:04PM +0800, Huang, Ying wrote:
> Hi, Byungchul,
>
> Byungchul Park <byungchul@sk.com> writes:
>
> > On Mon, May 13, 2024 at 10:44:29AM +0900, Byungchul Park wrote:
> >> On Sat, May 11, 2024 at 03:15:01PM +0800, Huang, Ying wrote:
> >> > Byungchul Park <byungchul@sk.com> writes:
> >> >
> >> > > Hi everyone,
> >> > >
> >> > > While I'm working with a tiered memory system e.g. CXL memory, I have
> >> > > been facing migration overhead esp. tlb shootdown on promotion or
> >> > > demotion between different tiers. Yeah.. most tlb shootdowns on
> >> > > migration through hinting fault can be avoided thanks to Huang Ying's
> >> > > work, commit 4d4b6d66db ("mm,unmap: avoid flushing tlb in batch if PTE
> >> > > is inaccessible"). See the following link for more information:
> >> > >
> >> > > https://lore.kernel.org/lkml/20231115025755.GA29979@system.software.com/
> >> >
> >> > And, I still have interest of the performance impact of commit
> >> > 7e12beb8ca2a ("migrate_pages: batch flushing TLB"). In the email above,
> >> > you said that the performance of v6.5-rc5 + 7e12beb8ca2a reverted has
> >> > better performance than v6.5-rc5. Can you provide more details? For
> >> > example, the number of TLB flushing IPI for two kernels?
> >>
> >> Okay. I will test and share the result with what you asked me now once
> >> I get available for the test.
> >
> > I should admit that the test using qemu is so unstable. While using
> > qemu for the test, kernel with 7e12beb8ca2a applied gave better results
> > sometimes and worse ones sometimes. I should've used a bare metal from
> > the beginning. Sorry for making you confused with the unstable result.
> >
> > Since I thought you asked me for the test with the same environment in
> > the link above, I used qemu to reproduce the similar result but changed
> > the number of threads for the test from 16 to 14 to get rid of noise
> > that might be introduced by other than the intended test just in case.
> >
> > As expected, the stats are better with your work:
> >
> > ------------------------------------------
> > v6.6-rc5 with 7e12beb8ca2a commit reverted
> > ------------------------------------------
> >
> > 1) from output of XSBench
> >
> > Threads: 14
> > Runtime: 1127.043 seconds
> > Lookups: 1,700,000,000
> > Lookups/s: 1,508,371
> >
> > 2) from /proc/vmstat
> >
> > numa_hit 15580171
> > numa_miss 1034233
> > numa_foreign 1034233
> > numa_interleave 773
> > numa_local 7927442
> > numa_other 8686962
> > numa_pte_updates 24068923
> > numa_hint_faults 24061125
> > numa_hint_faults_local 0
> > numa_pages_migrated 7426480
> > pgmigrate_success 15407375
> > pgmigrate_fail 1849
> > compact_migrate_scanned 4445414
> > compact_daemon_migrate_scanned 4445414
> > pgdemote_kswapd 7651061
> > pgdemote_direct 0
> > nr_tlb_remote_flush 8080092
> > nr_tlb_remote_flush_received 109915713
> > nr_tlb_local_flush_all 53800
> > nr_tlb_local_flush_one 770466
> >
> > 3) from /proc/interrupts
> >
> > TLB: 8022927 7840769 123588 7837008 7835967 7839837
> > 7838332 7839886 7837610 7837221 7834524 407260
> > 7430090 7835696 7839081 7712568 TLB shootdowns
> >
> > 4) from 'perf stat -a'
> >
> > 222371217 itlb.itlb_flush
> > 919832520 tlb_flush.dtlb_thread
> > 372223809 tlb_flush.stlb_any
> > 120210808042 dTLB-load-misses
> > 979352769 dTLB-store-misses
> > 3650767665 iTLB-load-misses
> >
> > -----------------------------------------
> > v6.6-rc5 with 7e12beb8ca2a commit applied
> > -----------------------------------------
> >
> > 1) from output of XSBench
> >
> > Threads: 14
> > Runtime: 1105.521 seconds
> > Lookups: 1,700,000,000
> > Lookups/s: 1,537,737
> >
> > 2) from /proc/vmstat
> >
> > numa_hit 24148399
> > numa_miss 797483
> > numa_foreign 797483
> > numa_interleave 772
> > numa_local 12214575
> > numa_other 12731307
> > numa_pte_updates 24250278
> > numa_hint_faults 24199756
> > numa_hint_faults_local 0
> > numa_pages_migrated 11476195
> > pgmigrate_success 23634639
> > pgmigrate_fail 1391
> > compact_migrate_scanned 3760803
> > compact_daemon_migrate_scanned 3760803
> > pgdemote_kswapd 11932217
> > pgdemote_direct 0
> > nr_tlb_remote_flush 2151945
> > nr_tlb_remote_flush_received 29672808
> > nr_tlb_local_flush_all 124006
> > nr_tlb_local_flush_one 741165
> >
> > 3) from /proc/interrupts
> >
> > TLB: 2130784 2120142 2117571 844962 2071766 114675
> > 2117258 2119596 2116816 1205446 2119176 2119209
> > 2116792 2118763 2118773 2117762 TLB shootdowns
> >
> > 4) from 'perf stat -a'
> >
> > 60851902 itlb.itlb_flush
> > 334068491 tlb_flush.dtlb_thread
> > 223732916 tlb_flush.stlb_any
> > 120207083382 dTLB-load-misses
> > 446823059 dTLB-store-misses
> > 1926669373 iTLB-load-misses
> >
>
> Thanks a lot for test results!
>
> >From your test results, the TLB shootdown IPI can be reduced effectively
> with commit 7e12beb8ca2a. So that the benchmark score improved a
> little.
>
> And, your changes will reduce the TLB shootdown IPI further, right? Do

Yes, right. LUF(Lazy Unmap Flush) reduces TLB shootdown IPI further.

> you have the number?

You can find the number obtained from llama.cpp in this cover letter:

https://lore.kernel.org/lkml/20240520021734.21527-1-byungchul@sk.com/

If you meant the number from the same test above, XSBench + qemu, I will
re-test with mm-unstable branch of mm tree and share the result shortly.

Byungchul