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Message-ID: <YHmcorqNE5NpAN3G@google.com>
Date: Fri, 16 Apr 2021 14:18:10 +0000
From: Dennis Zhou <dennis@...nel.org>
To: Pratik Sampat <psampat@...ux.ibm.com>
Cc: Roman Gushchin <guro@...com>, Tejun Heo <tj@...nel.org>,
Christoph Lameter <cl@...ux.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, pratik.r.sampat@...il.com
Subject: Re: [PATCH v3 0/6] percpu: partial chunk depopulation
Hello,
On Fri, Apr 16, 2021 at 06:26:15PM +0530, Pratik Sampat wrote:
> Hello Roman,
>
> I've tried the v3 patch series on a POWER9 and an x86 KVM setup.
>
> My results of the percpu_test are as follows:
> Intel KVM 4CPU:4G
> Vanilla 5.12-rc6
> # ./percpu_test.sh
> Percpu: 1952 kB
> Percpu: 219648 kB
> Percpu: 219648 kB
>
> 5.12-rc6 + with patchset applied
> # ./percpu_test.sh
> Percpu: 2080 kB
> Percpu: 219712 kB
> Percpu: 72672 kB
>
> I'm able to see improvement comparable to that of what you're see too.
>
> However, on POWERPC I'm unable to reproduce these improvements with the patchset in the same configuration
>
> POWER9 KVM 4CPU:4G
> Vanilla 5.12-rc6
> # ./percpu_test.sh
> Percpu: 5888 kB
> Percpu: 118272 kB
> Percpu: 118272 kB
>
> 5.12-rc6 + with patchset applied
> # ./percpu_test.sh
> Percpu: 6144 kB
> Percpu: 119040 kB
> Percpu: 119040 kB
>
> I'm wondering if there's any architectural specific code that needs plumbing
> here?
>
There shouldn't be. Can you send me the percpu_stats debug output before
and after?
> I will also look through the code to find the reason why POWER isn't
> depopulating pages.
>
> Thank you,
> Pratik
>
> On 08/04/21 9:27 am, Roman Gushchin wrote:
> > In our production experience the percpu memory allocator is sometimes struggling
> > with returning the memory to the system. A typical example is a creation of
> > several thousands memory cgroups (each has several chunks of the percpu data
> > used for vmstats, vmevents, ref counters etc). Deletion and complete releasing
> > of these cgroups doesn't always lead to a shrinkage of the percpu memory,
> > so that sometimes there are several GB's of memory wasted.
> >
> > The underlying problem is the fragmentation: to release an underlying chunk
> > all percpu allocations should be released first. The percpu allocator tends
> > to top up chunks to improve the utilization. It means new small-ish allocations
> > (e.g. percpu ref counters) are placed onto almost filled old-ish chunks,
> > effectively pinning them in memory.
> >
> > This patchset solves this problem by implementing a partial depopulation
> > of percpu chunks: chunks with many empty pages are being asynchronously
> > depopulated and the pages are returned to the system.
> >
> > To illustrate the problem the following script can be used:
> >
> > --
> > #!/bin/bash
> >
> > cd /sys/fs/cgroup
> >
> > mkdir percpu_test
> > echo "+memory" > percpu_test/cgroup.subtree_control
> >
> > cat /proc/meminfo | grep Percpu
> >
> > for i in `seq 1 1000`; do
> > mkdir percpu_test/cg_"${i}"
> > for j in `seq 1 10`; do
> > mkdir percpu_test/cg_"${i}"_"${j}"
> > done
> > done
> >
> > cat /proc/meminfo | grep Percpu
> >
> > for i in `seq 1 1000`; do
> > for j in `seq 1 10`; do
> > rmdir percpu_test/cg_"${i}"_"${j}"
> > done
> > done
> >
> > sleep 10
> >
> > cat /proc/meminfo | grep Percpu
> >
> > for i in `seq 1 1000`; do
> > rmdir percpu_test/cg_"${i}"
> > done
> >
> > rmdir percpu_test
> > --
> >
> > It creates 11000 memory cgroups and removes every 10 out of 11.
> > It prints the initial size of the percpu memory, the size after
> > creating all cgroups and the size after deleting most of them.
> >
> > Results:
> > vanilla:
> > ./percpu_test.sh
> > Percpu: 7488 kB
> > Percpu: 481152 kB
> > Percpu: 481152 kB
> >
> > with this patchset applied:
> > ./percpu_test.sh
> > Percpu: 7488 kB
> > Percpu: 481408 kB
> > Percpu: 135552 kB
> >
> > So the total size of the percpu memory was reduced by more than 3.5 times.
> >
> > v3:
> > - introduced pcpu_check_chunk_hint()
> > - fixed a bug related to the hint check
> > - minor cosmetic changes
> > - s/pretends/fixes (cc Vlastimil)
> >
> > v2:
> > - depopulated chunks are sidelined
> > - depopulation happens in the reverse order
> > - depopulate list made per-chunk type
> > - better results due to better heuristics
> >
> > v1:
> > - depopulation heuristics changed and optimized
> > - chunks are put into a separate list, depopulation scan this list
> > - chunk->isolated is introduced, chunk->depopulate is dropped
> > - rearranged patches a bit
> > - fixed a panic discovered by krobot
> > - made pcpu_nr_empty_pop_pages per chunk type
> > - minor fixes
> >
> > rfc:
> > https://lwn.net/Articles/850508/
> >
> >
> > Roman Gushchin (6):
> > percpu: fix a comment about the chunks ordering
> > percpu: split __pcpu_balance_workfn()
> > percpu: make pcpu_nr_empty_pop_pages per chunk type
> > percpu: generalize pcpu_balance_populated()
> > percpu: factor out pcpu_check_chunk_hint()
> > percpu: implement partial chunk depopulation
> >
> > mm/percpu-internal.h | 4 +-
> > mm/percpu-stats.c | 9 +-
> > mm/percpu.c | 306 +++++++++++++++++++++++++++++++++++--------
> > 3 files changed, 261 insertions(+), 58 deletions(-)
> >
>
Roman, sorry for the delay. I'm looking to apply this today to for-5.14.
Thanks,
Dennis
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