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

[Ryan Roberts <ryan.roberts@arm.com>]

On 30/05/2024 08:49, Barry Song wrote:
> On Wed, May 29, 2024 at 9:04 AM Chris Li <chrisl@kernel.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 :-)

No problem; clearly it needs to be fixed, and I'll help where I can. I'm pretty
sure that this is due to fragmentation preventing clusters from being freed back
to the free list.

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

Unless I've misunderstood something critical, Chris's change is just allowing a
cpu to steal a block from another cpu's current cluster for that order. So it
just takes longer (approx by a factor of the number of cpus in the system) to
get to the state where fragmentation is causing fallbacks? As I said in the
other thread, I think the more robust solution is to implement scanning for high
order blocks.

> 
> 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@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>
>>>
> 
> Thanks
> Barry