Re: [RFC 0/2] ext4: Improve locking sequence in DIO write path

Ritesh Harjani
2019-09-18T10:03:47Z

Hello Joseph,

First of all thanks a lot for collecting a thorough
performance numbers.

On 9/18/19 12:05 PM, Joseph Qi wrote:
> Hi Ritesh,
>
> On 19/9/17 18:32, Ritesh Harjani wrote:
>> Hello,
>>
>> This patch series is based on the upstream discussion with Jan
>> & Joseph @ [1].
>> It is based on top of Matthew's v3 ext4 iomap patch series [2]
>>
>> Patch-1: Adds the ext4_ilock/unlock APIs and also replaces all
>> inode_lock/unlock instances from fs/ext4/*
>>
>> For now I already accounted for trylock/lock issue symantics
>> (which was discussed here [3]) in the same patch,
>> since the this whole patch was around inode_lock/unlock API,
>> so I thought it will be best to address that issue in the same patch.
>> However, kindly let me know if otherwise.
>>
>> Patch-2: Commit msg of this patch describes in detail about
>> what it is doing.
>> In brief - we try to first take the shared lock (instead of exclusive
>> lock), unless it is a unaligned_io or extend_io. Then in
>> ext4_dio_write_checks(), if we start with shared lock, we see
>> if we can really continue with shared lock or not. If not, then
>> we release the shared lock then acquire exclusive lock
>> and restart ext4_dio_write_checks().
>>
>>
>> Tested against few xfstests (with dioread_nolock mount option),
>> those ran fine (ext4 & generic).
>>
>> I tried testing performance numbers on my VM (since I could not get
>> hold of any real h/w based test device). I could test the fact
>> that earlier we were trying to do downgrade_write() lock, but with
>> this patch, that path is now avoided for fio test case
>> (as reported by Joseph in [4]).
>> But for the actual results, I am not sure if VM machine testing could
>> really give the reliable perf numbers which we want to take a look at.
>> Though I do observe some form of perf improvements, but I could not
>> get any reliable numbers (not even with the same list of with/without
>> patches with which Joseph posted his numbers [1]).
>>
>>
>> @Joseph,
>> Would it be possible for you to give your test case a run with this
>> patches? That will be really helpful.
>>
>> Branch for this is hosted at below tree.
>>
>> https://github.com/riteshharjani/linux/tree/ext4-ilock-RFC
>>
> I've tested your branch, the result is:
> mounting with dioread_nolock, it behaves the same like reverting
> parallel dio reads + dioread_nolock;

Good sign, means that patch is doing what it is supposed to do.

> while mounting without dioread_nolock, no improvement, or even worse.
> Please refer the test data below.
Actually without dioread_nolock, we take the restart path.
i.e. initially we start with SHARED_LOCK, but if dioread_nolock
is not enabled (or check some other conditions like overwrite),
we release the shared lock and re-acquire the EXCL lock.

But as an optimization, I added the below diff just now
to directly first check for ext4_should_dioread_nolock too
before taking the shared lock.

I think with this we should not see any performance regression
(even without dioread_nolock mount option).
Since it will directly start with exclusive lock
if dioread_nolock mount option is not enabled.

I have updated the tree with this diff in same branch.

ext4_dio_file_write_iter ()
<...>

498 if (iolock == EXT4_IOLOCK_SHARED &&
!ext4_should_dioread_nolock(inode))
499 iolock = EXT4_IOLOCK_EXCL;
500
<...>

>
> fio -name=parallel_dio_reads_test -filename=/mnt/nvme0n1/testfile
> -direct=1 -iodepth=1 -thread -rw=randrw -ioengine=psync -bs=$bs
> -size=20G -numjobs=8 -runtime=600 -group_reporting
>
> w/ = with parallel dio reads
> w/o = reverting parallel dio reads
> w/o+ = reverting parallel dio reads + dioread_nolock
> ilock = ext4-ilock-RFC
> ilock+ = ext4-ilock-RFC + dioread_nolock

I will request to kindly also add "w/ + dioread_nolock" in your list.

"w/ + dioread_nolock" v/s "ilock+" - should show some improvements.
"w/ " v/s "ilock" - should not show any regression.

But thanks for the exhaustive performance numbers you collected.

-ritesh

>
> bs=4k:
> --------------------------------------------------------------
> | READ | WRITE |
> --------------------------------------------------------------
> w/ | 30898KB/s,7724,555.00us | 30875KB/s,7718,479.70us |
> --------------------------------------------------------------
> w/o | 117915KB/s,29478,248.18us | 117854KB/s,29463,21.91us |
> --------------------------------------------------------------
> w/o+ | 123450KB/s,30862,245.77us | 123368KB/s,30841,12.14us |
> --------------------------------------------------------------
> ilock | 29964KB/s,7491,326.70us | 29940KB/s,7485,740.62us |
> --------------------------------------------------------------
> ilock+| 123685KB/s,30921,245.52us | 123601KB/s,30900,12.11us |
> --------------------------------------------------------------
>
>
> bs=16k:
> --------------------------------------------------------------
> | READ | WRITE |
> --------------------------------------------------------------
> w/ | 58961KB/s,3685,835.28us | 58877KB/s,3679,1335.98us |
> --------------------------------------------------------------
> w/o | 218409KB/s,13650,554.46us | 218257KB/s,13641,29.22us |
> --------------------------------------------------------------
> w/o+ | 222477KB/s,13904,552.94us | 222322KB/s,13895,20.28us |
> --------------------------------------------------------------
> ilock | 56039KB/s,3502,632.96us | 55943KB/s,3496,1652.72us |
> --------------------------------------------------------------
> ilock+| 222747KB/s,13921,552.57us | 222592KB/s,13912,20.31us |
> --------------------------------------------------------------
>
> bs=64k
> ----------------------------------------------------------------
> | READ | WRITE |
> ----------------------------------------------------------------
> w/ | 119396KB/s,1865,1759.38us | 119159KB/s,1861,2532.26us |
> ----------------------------------------------------------------
> w/o | 422815KB/s,6606,1146.05us | 421619KB/s,6587,60.72us |
> --------------------------------------------,-------------------
> w/o+ | 427406KB/s,6678,1141.52us | 426197KB/s,6659,52.79us |
> ----------------------------------------------------------------
> ilock | 105800KB/s,1653,1451.68us | 105721KB/s,1651,3388.64us |
> ----------------------------------------------------------------
> ilock+| 427678KB/s,6682,1142.13us | 426468KB/s,6663,52.31us |
> ----------------------------------------------------------------
>
> bs=512k
> ----------------------------------------------------------------
> | READ | WRITE |
> ----------------------------------------------------------------
> w/ | 392973KB/s,767,5046.35us | 393165KB/s,767,5359.86us |
> ----------------------------------------------------------------
> w/o | 590266KB/s,1152,4312.01us | 590554KB/s,1153,2606.82us |
> ----------------------------------------------------------------
> w/o+ | 618752KB/s,1208,4125.82us | 619054KB/s,1209,2487.90us |
> ----------------------------------------------------------------
> ilock | 296239KB/s,578,4703.10us | 296384KB/s,578,9049.32us |
> ----------------------------------------------------------------
> ilock+| 616636KB/s,1204,4143.38us | 616937KB/s,1204,2490.08us |
> ----------------------------------------------------------------
>
> bs=1M
> ----------------------------------------------------------------
> | READ | WRITE |
> ----------------------------------------------------------------
> w/ | 487779KB/s,476,8058.55us | 485592KB/s,474,8630.51us |
> ----------------------------------------------------------------
> w/o | 593927KB/s,580,7623.63us | 591265KB/s,577,6163.42us |
> ----------------------------------------------------------------
> w/o+ | 615011KB/s,600,7399.93us | 612255KB/s,597,5936.61us |
> ----------------------------------------------------------------
> ilock | 394762KB/s,385,7097.55us | 392993KB/s,383,13626.98us |
> ----------------------------------------------------------------
> ilock+| 626183KB/s,611,7319.16us | 623377KB/s,608,5773.24us |
> ----------------------------------------------------------------
>
> Thanks,
> Joseph
>