On Fri, Apr 19, 2024 at 02:06:30PM +0800, Huang, Ying wrote:
> Byungchul Park <byungchul@xxxxxx> 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@xxxxxxxxxxxxxxxxxxx/
> >
> > However, it's only for ones using hinting fault. I thought it'd be much
> > better if we have a general mechanism to reduce all tlb numbers that we
> > can ultimately apply to any type of migration.
> >
> > I'm suggesting a mechanism called MIGRC that stands for 'Migration Read
> > Copy', to reduce tlb numbers by deferring tlb flush until the source
> > folios at migration actually become used, of course, only if the target
> > PTE don't have write permission.
> >
> > To achieve that:
> >
> > 1. For the folios that map only to non-writable tlb entries, prevent
> > tlb flush during migration but perform it just before the source
> > folios actually become used out of buddy or pcp.
> >
> > 2. When any non-writable tlb entry changes to writable e.g. through
> > fault handler, give up migrc mechanism and perform tlb flush
> > required right away.
> >
> > No matter what type of workload is used for performance evaluation, the
> > result would be positive thanks to the unconditional reduction of tlb
> > flushes, tlb misses and interrupts. For the test, I picked up XSBench
> > that is widely used for performance analysis on high performance
> > computing architectures - https://github.com/ANL-CESAR/XSBench.
> >
> > The result would depend on memory latency and how often reclaim runs,
> > which implies tlb miss overhead and how many times migration happens.
> > The slower the memory is and the more reclaim runs, the better migrc
> > works so as to obtain the better result. In my system, the result
> > shows:
> >
> > 1. itlb flushes are reduced over 90%.
> > 2. itlb misses are reduced over 30%.
> > 3. All the other tlb numbers also get enhanced.
> > 4. tlb shootdown interrupts are reduced over 90%.
> > 5. The test program runtime is reduced over 5%.
> >
> > The test envitonment:
> >
> > Architecture - x86_64
> > QEMU - kvm enabled, host cpu
>
> The test is run in VM? Do you have test results in bare metal
> environment?
I will test in a bare metal environment and share the result.
> > Numa - 2 nodes (16 CPUs 1GB, no CPUs 99GB)
>
> The configuration looks quite abnormal. Have you tested with other
> configuration, such 1:4 or 1:8?
Okay I will test with the configurations.
> > Linux Kernel - v6.9-rc4, numa balancing tiering on, demotion enabled
> >
> > < measurement: raw data - tlb and interrupt numbers >
> >
> > $ perf stat -a \
> > -e itlb.itlb_flush \
> > -e tlb_flush.dtlb_thread \
> > -e tlb_flush.stlb_any \
> > -e dtlb-load-misses \
> > -e dtlb-store-misses \
> > -e itlb-load-misses \
> > XSBench -t 16 -p 50000000
> >
> > $ grep "TLB shootdowns" /proc/interrupts
> >
> > BEFORE
> > ------
> > 40417078 itlb.itlb_flush
> > 234852566 tlb_flush.dtlb_thread
> > 153192357 tlb_flush.stlb_any
> > 119001107892 dTLB-load-misses
> > 307921167 dTLB-store-misses
> > 1355272118 iTLB-load-misses
> >
> > TLB: 1364803 1303670 1333921 1349607
> > 1356934 1354216 1332972 1342842
> > 1350265 1316443 1355928 1360793
> > 1298239 1326358 1343006 1340971
> > TLB shootdowns
> >
> > AFTER
> > -----
> > 3316495 itlb.itlb_flush
> > 138912511 tlb_flush.dtlb_thread
> > 115199341 tlb_flush.stlb_any
> > 117610390021 dTLB-load-misses
> > 198042233 dTLB-store-misses
> > 840066984 iTLB-load-misses
> >
> > TLB: 117257 119219 117178 115737
> > 117967 118948 117508 116079
> > 116962 117266 117320 117215
> > 105808 103934 115672 117610
> > TLB shootdowns
> >
> > < measurement: user experience - runtime >
> >
> > $ time XSBench -t 16 -p 50000000
> >
> > BEFORE
> > ------
> > Threads: 16
> > Runtime: 968.783 seconds
> > Lookups: 1,700,000,000
> > Lookups/s: 1,754,778
> >
> > 15208.91s user 141.44s system 1564% cpu 16:20.98 total
> >
> > AFTER
> > -----
> > Threads: 16
> > Runtime: 913.210 seconds
> > Lookups: 1,700,000,000
> > Lookups/s: 1,861,565
> >
> > 14351.69s user 138.23s system 1565% cpu 15:25.47 total
>
> IIUC, the memory footprint will be larger with the patchset. Do you
> have data?
No. The footprint is, I expect, same as vanilla with this patchset. I
will share the data.
Last time, since you pointed out that the footprint seemed to be larger
with the previous patchset becasue it worked anyway based on deferring
freeing folios.
Which made me rework on it so as to avoid tweaking the original behavior
of mm. Instead, the current version of migrc let it go exactly same as
it is with vanilla until the interesting folios exit from pcp or buddy,
and do tlb flush if needed.
Byungchul
> --
> Best Regards,
> Huang, Ying
