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

Joseph Qi
2019-09-19T02:08:23Z

On 19/9/18 18:03, Ritesh Harjani wrote:
> 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
> <...>
>
With this optimization, when mounting without dioread_nolock, it has
a little improvement compared to the last ilock version, but still
poor than original, especially for big block size.

w/ = with parallel dio reads (original)
w/o = reverting parallel dio reads
w/o+ = reverting parallel dio reads + dioread_nolock
ilock = ext4-ilock-RFC
ilock+ = ext4-ilock-RFC + dioread_nolock
ilocknew = ext4-ilock-RFC latest
ilocknew+ = ext4-ilock-RFC latest + dioread_nolock

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 |
------------------------------------------------------------------
ilocknew | 30190KB/s,7547,497.47us | 30159KB/s,7539,561.85us |
------------------------------------------------------------------
ilock+ | 123685KB/s,30921,245.52us | 123601KB/s,30900,12.11us |
------------------------------------------------------------------
ilocknew+ | 123169KB/s,30792,245.81us | 123087KB/s,30771,12.85us |
------------------------------------------------------------------

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 |
------------------------------------------------------------------
ilocknew | 57317KB/s,3582,1023.88us | 57230KB/s,3576,1209.91us |
------------------------------------------------------------------
ilock+ | 222747KB/s,13921,552.57us | 222592KB/s,13912,20.31us |
------------------------------------------------------------------
ilocknew+ | 221945KB/s,13871,552.61us | 221791KB/s,13861,21.29us |
------------------------------------------------------------------

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 |
-------------------------------------------------------------------
ilocknew | 107447KB/s,1678,1654.33us | 107322KB/s,1676,3112.96us |
-------------------------------------------------------------------
ilock+ | 427678KB/s,6682,1142.13us | 426468KB/s,6663,52.31us |
-------------------------------------------------------------------
ilocknew+ | 427054KB/s,6672,1143.43us | 425846KB/s,6653,52.87us |
-------------------------------------------------------------------

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 |
-------------------------------------------------------------------
ilocknew | 309740KB/s,604,5485.93us | 309892KB/s,605,7666.79us |
-------------------------------------------------------------------
ilock+ | 616636KB/s,1204,4143.38us | 616937KB/s,1204,2490.08us |
-------------------------------------------------------------------
ilocknew+ | 618159KB/s,1207,4129.76us | 618461KB/s,1207,2486.90us |
-------------------------------------------------------------------

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 |
-------------------------------------------------------------------
ilocknew | 422052KB/s,412,8338.16us | 420161KB/s,410,11008.95us |
-------------------------------------------------------------------
ilock+ | 626183KB/s,611,7319.16us | 623377KB/s,608,5773.24us |
-------------------------------------------------------------------
ilocknew+ | 626006KB/s,611,7281.09us | 623200KB/s,608,5817.84us |
-------------------------------------------------------------------