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

Ritesh Harjani
2019-09-19T18:48:52Z

Hello,

On 9/19/19 7:38 AM, Joseph Qi wrote:
>
>
> 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.

Finally, I got hold of some HW. I am collecting the numbers as we speak.
Will post those tomorrow.

Thanks for your help!!

-ritesh

>
> 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 |
> -------------------------------------------------------------------
>