Re: [PATCH 0/2] mm: swap: mTHP swap allocator base on swap cluster order

["Huang, Ying" <ying.huang@intel.com>]

Chris Li <chrisl@kernel.org> writes:

> 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.

IIUC, now, we only scan swap_map[] only if
!list_empty(&si->free_clusters) && !list_empty(&si->nonfull_clusters[order]).
That is, if you doesn't run low swap free space, you will not do that.

> 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.

Better to do any further optimization on top of the simpler one.  Need
to evaluate whether it's necessary to add more complexity.

> 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.

I have thought about that too.  Personally, I think that it's good to
remove swap_map[] scanning.  The implementation can be simplified too.
I don't know whether do we need to consider the performance of HDD swap
now.

--
Best Regards,
Huang, Ying

> On Fri, May 24, 2024 at 10:17 AM Chris Li <chrisl@kernel.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@gmail.com>
>> Tested-by: Kairui Song <kasong@tencent.com>
>> Signed-off-by: Chris Li <chrisl@kernel.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@kernel.org>
>>