Messages in this thread | | | Date | Tue, 28 Feb 2023 18:04:53 +0800 | Subject | Re: [PATCH v3 0/8] make slab shrink lockless | From | Qi Zheng <> |
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On 2023/2/27 23:08, Mike Rapoport wrote: > Hi, > > On Mon, Feb 27, 2023 at 09:31:51PM +0800, Qi Zheng wrote: >> >> >> On 2023/2/27 03:51, Andrew Morton wrote: >>> On Sun, 26 Feb 2023 22:46:47 +0800 Qi Zheng <zhengqi.arch@bytedance.com> wrote: >>> >>>> Hi all, >>>> >>>> This patch series aims to make slab shrink lockless. >>> >>> What an awesome changelog. >>> >>>> 2. Survey >>>> ========= >>> >>> Especially this part. >>> >>> Looking through all the prior efforts and at this patchset I am not >>> immediately seeing any statements about the overall effect upon >>> real-world workloads. For a good example, does this patchset >>> measurably improve throughput or energy consumption on your servers? >> >> Hi Andrew, >> >> I re-tested with the following physical machines: >> >> Architecture: x86_64 >> CPU(s): 96 >> On-line CPU(s) list: 0-95 >> Model name: Intel(R) Xeon(R) Platinum 8260 CPU @ 2.40GHz >> >> I found that the reason for the hotspot I described in cover letter is >> wrong. The reason for the down_read_trylock() hotspot is not because of >> the failure to trylock, but simply because of the atomic operation >> (cmpxchg). And this will lead to a significant reduction in IPC (insn >> per cycle). > > ... > >> Then we can use the following perf command to view hotspots: >> >> perf top -U -F 999 >> >> 1) Before applying this patchset: >> >> 32.31% [kernel] [k] down_read_trylock >> 19.40% [kernel] [k] pv_native_safe_halt >> 16.24% [kernel] [k] up_read >> 15.70% [kernel] [k] shrink_slab >> 4.69% [kernel] [k] _find_next_bit >> 2.62% [kernel] [k] shrink_node >> 1.78% [kernel] [k] shrink_lruvec >> 0.76% [kernel] [k] do_shrink_slab >> >> 2) After applying this patchset: >> >> 27.83% [kernel] [k] _find_next_bit >> 16.97% [kernel] [k] shrink_slab >> 15.82% [kernel] [k] pv_native_safe_halt >> 9.58% [kernel] [k] shrink_node >> 8.31% [kernel] [k] shrink_lruvec >> 5.64% [kernel] [k] do_shrink_slab >> 3.88% [kernel] [k] mem_cgroup_iter >> >> 2. At the same time, we use the following perf command to capture IPC >> information: >> >> perf stat -e cycles,instructions -G test -a --repeat 5 -- sleep 10 >> >> 1) Before applying this patchset: >> >> Performance counter stats for 'system wide' (5 runs): >> >> 454187219766 cycles test ( >> +- 1.84% ) >> 78896433101 instructions test # 0.17 insn per >> cycle ( +- 0.44% ) >> >> 10.0020430 +- 0.0000366 seconds time elapsed ( +- 0.00% ) >> >> 2) After applying this patchset: >> >> Performance counter stats for 'system wide' (5 runs): >> >> 841954709443 cycles test ( >> +- 15.80% ) (98.69%) >> 527258677936 instructions test # 0.63 insn per >> cycle ( +- 15.11% ) (98.68%) >> >> 10.01064 +- 0.00831 seconds time elapsed ( +- 0.08% ) >> >> We can see that IPC drops very seriously when calling >> down_read_trylock() at high frequency. After using SRCU, >> the IPC is at a normal level. > > The results you present do show improvement in IPC for an artificial test > script. But more interesting would be to see how a real world workloads > benefit from your changes.
Hi Mike and Andrew,
I did encounter this problem under the real workload of our online server. At the end of this email, I posted another call stack and hot spot that I found before.
I scanned the hotspots of all our online servers yesterday and today, but unfortunately did not find the live environment.
Some of our servers have a large number of containers, and each container will mount some file systems. This is likely to trigger down_read_trylock() hotspots when the memory pressure of the whole machine or the memory pressure of memcg is high.
So I just found a physical server with a similar configuration to the online server yesterday for a simulation test. The call stack and the hot spot in the simulation test are almost exactly the same, so in theory, when such a hot spot appears on the online server, we can also enjoy the improvement of IPC. This will improve the performance of the server in memory exhaustion scenarios (memcg or global level).
And the above scenario is only one aspect, and the other aspect is the lock competition scenario mentioned by Kirill. After applying this patch set, slab shrink and register_shrinker() can be completely parallelized, which can fix that problem.
These are the two main benefits for real workloads that I consider.
Thanks, Qi
call stack ----------
@[ down_read_trylock+1 shrink_slab+128 shrink_node+371 do_try_to_free_pages+232 try_to_free_pages+243 _alloc_pages_slowpath+771 _alloc_pages_nodemask+702 pagecache_get_page+255 filemap_fault+1361 ext4_filemap_fault+44 __do_fault+76 handle_mm_fault+3543 do_user_addr_fault+442 do_page_fault+48 page_fault+62 ]: 1161690 @[ down_read_trylock+1 shrink_slab+128 shrink_node+371 balance_pgdat+690 kswapd+389 kthread+246 ret_from_fork+31 ]: 8424884 @[ down_read_trylock+1 shrink_slab+128 shrink_node+371 do_try_to_free_pages+232 try_to_free_pages+243 __alloc_pages_slowpath+771 __alloc_pages_nodemask+702 __do_page_cache_readahead+244 filemap_fault+1674 ext4_filemap_fault+44 __do_fault+76 handle_mm_fault+3543 do_user_addr_fault+442 do_page_fault+48 page_fault+62 ]: 20917631
hotspot -------
52.22% [kernel] [k] down_read_trylock 19.60% [kernel] [k] up_read 8.86% [kernel] [k] shrink_slab 2.44% [kernel] [k] idr_find 1.25% [kernel] [k] count_shadow_nodes 1.18% [kernel] [k] shrink lruvec 0.71% [kernel] [k] mem_cgroup_iter 0.71% [kernel] [k] shrink_node 0.55% [kernel] [k] find_next_bit
> >> Thanks, >> Qi >
-- Thanks, Qi
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