Date: Thu, 30 May 2024 19:49:19 +1200
From: Barry Song <baohua@...nel.org>
To: Chris Li <chrisl@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Kairui Song <kasong@...cent.com>, 
	Ryan Roberts <ryan.roberts@....com>, "Huang, Ying" <ying.huang@...el.com>, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 0/2] mm: swap: mTHP swap allocator base on swap cluster order

On Wed, May 29, 2024 at 9:04 AM Chris Li <chrisl@...nel.org> wrote:
>
> I am spinning a new version for this series to address two issues
> found in this series:
>
> 1) Oppo discovered a bug in the following line:
> +               ci = si->cluster_info + tmp;
> Should be "tmp / SWAPFILE_CLUSTER" instead of "tmp".
> That is a serious bug but trivial to fix.
>
> 2) order 0 allocation currently blindly scans swap_map disregarding
> the cluster->order. Given enough order 0 swap allocations(close to the
> swap file size) the order 0 allocation head will eventually sweep
> across the whole swapfile and destroy other cluster order allocations.
>
> The short term fix is just skipping clusters that are already assigned
> to higher orders.
>
> In the long term, I want to unify the non-SSD to use clusters for
> locking and allocations as well, just try to follow the last
> allocation (less seeking) as much as possible.

Hi Chris,

I am sharing some new test results with you. This time, we used two
zRAM devices by modifying get_swap_pages().

zram0 -> dedicated for order-0 swpout
zram1 -> dedicated for order-4 swpout

We allocate a generous amount of space for zRAM1 to ensure it never gets full
and always has ample free space. However, we found that Ryan's approach
does not perform well even in this straightforward scenario. Despite zRAM1
having 80% of its space remaining, we still experience issues obtaining
contiguous swap slots and encounter a high swpout_fallback ratio.

Sorry for the report, Ryan :-)

In contrast, with your patch, we consistently see the thp_swpout_fallback ratio
at 0%, indicating a significant improvement in the situation.

Although your patch still has issues supporting the mixing of order-0 and
order-4 pages in a swap device, it represents a significant improvement.

I would be delighted to witness your approach advancing with Ying
Huang’s assistance. However, due to my current commitments, I
regret that I am unable to allocate time for debugging.

>
> Chris
>
>
>
> On Fri, May 24, 2024 at 10:17 AM Chris Li <chrisl@...nel.org> wrote:
> >
> > This is the short term solutiolns "swap cluster order" listed
> > in my "Swap Abstraction" discussion slice 8 in the recent
> > LSF/MM conference.
> >
> > When commit 845982eb264bc "mm: swap: allow storage of all mTHP
> > orders" is introduced, it only allocates the mTHP swap entries
> > from new empty cluster list. That works well for PMD size THP,
> > but it has a serius fragmentation issue reported by Barry.
> >
> > https://lore.kernel.org/all/CAGsJ_4zAcJkuW016Cfi6wicRr8N9X+GJJhgMQdSMp+Ah+NSgNQ@mail.gmail.com/
> >
> > The mTHP allocation failure rate raises to almost 100% after a few
> > hours in Barry's test run.
> >
> > The reason is that all the empty cluster has been exhausted while
> > there are planty of free swap entries to in the cluster that is
> > not 100% free.
> >
> > Address this by remember the swap allocation order in the cluster.
> > Keep track of the per order non full cluster list for later allocation.
> >
> > This greatly improve the sucess rate of the mTHP swap allocation.
> > While I am still waiting for Barry's test result. I paste Kairui's test
> > result here:
> >
> > I'm able to reproduce such an issue with a simple script (enabling all order of mthp):
> >
> > modprobe brd rd_nr=1 rd_size=$(( 10 * 1024 * 1024))
> > swapoff -a
> > mkswap /dev/ram0
> > swapon /dev/ram0
> >
> > rmdir /sys/fs/cgroup/benchmark
> > mkdir -p /sys/fs/cgroup/benchmark
> > cd /sys/fs/cgroup/benchmark
> > echo 8G > memory.max
> > echo $$ > cgroup.procs
> >
> > memcached -u nobody -m 16384 -s /tmp/memcached.socket -a 0766 -t 32 -B binary &
> >
> > /usr/local/bin/memtier_benchmark -S /tmp/memcached.socket \
> >         -P memcache_binary -n allkeys --key-minimum=1 \
> >         --key-maximum=18000000 --key-pattern=P:P -c 1 -t 32 \
> >         --ratio 1:0 --pipeline 8 -d 1024
> >
> > Before:
> > Totals      48805.63         0.00         0.00         5.26045         1.19100        38.91100        59.64700     51063.98
> > After:
> > Totals      71098.84         0.00         0.00         3.60585         0.71100        26.36700        39.16700     74388.74
> >
> > And the fallback ratio dropped by a lot:
> > Before:
> > hugepages-32kB/stats/anon_swpout_fallback:15997
> > hugepages-32kB/stats/anon_swpout:18712
> > hugepages-512kB/stats/anon_swpout_fallback:192
> > hugepages-512kB/stats/anon_swpout:0
> > hugepages-2048kB/stats/anon_swpout_fallback:2
> > hugepages-2048kB/stats/anon_swpout:0
> > hugepages-1024kB/stats/anon_swpout_fallback:0
> > hugepages-1024kB/stats/anon_swpout:0
> > hugepages-64kB/stats/anon_swpout_fallback:18246
> > hugepages-64kB/stats/anon_swpout:17644
> > hugepages-16kB/stats/anon_swpout_fallback:13701
> > hugepages-16kB/stats/anon_swpout:18234
> > hugepages-256kB/stats/anon_swpout_fallback:8642
> > hugepages-256kB/stats/anon_swpout:93
> > hugepages-128kB/stats/anon_swpout_fallback:21497
> > hugepages-128kB/stats/anon_swpout:7596
> >
> > (Still collecting more data, the success swpout was mostly done early, then the fallback began to increase, nearly 100% failure rate)
> >
> > After:
> > hugepages-32kB/stats/swpout:34445
> > hugepages-32kB/stats/swpout_fallback:0
> > hugepages-512kB/stats/swpout:1
> > hugepages-512kB/stats/swpout_fallback:134
> > hugepages-2048kB/stats/swpout:1
> > hugepages-2048kB/stats/swpout_fallback:1
> > hugepages-1024kB/stats/swpout:6
> > hugepages-1024kB/stats/swpout_fallback:0
> > hugepages-64kB/stats/swpout:35495
> > hugepages-64kB/stats/swpout_fallback:0
> > hugepages-16kB/stats/swpout:32441
> > hugepages-16kB/stats/swpout_fallback:0
> > hugepages-256kB/stats/swpout:2223
> > hugepages-256kB/stats/swpout_fallback:6278
> > hugepages-128kB/stats/swpout:29136
> > hugepages-128kB/stats/swpout_fallback:52
> >
> > Reported-by: Barry Song <21cnbao@...il.com>
> > Tested-by: Kairui Song <kasong@...cent.com>
> > Signed-off-by: Chris Li <chrisl@...nel.org>
> > ---
> > Chris Li (2):
> >       mm: swap: swap cluster switch to double link list
> >       mm: swap: mTHP allocate swap entries from nonfull list
> >
> >  include/linux/swap.h |  18 ++--
> >  mm/swapfile.c        | 252 +++++++++++++++++----------------------------------
> >  2 files changed, 93 insertions(+), 177 deletions(-)
> > ---
> > base-commit: c65920c76a977c2b73c3a8b03b4c0c00cc1285ed
> > change-id: 20240523-swap-allocator-1534c480ece4
> >
> > Best regards,
> > --
> > Chris Li <chrisl@...nel.org>
> >

Thanks
Barry

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