Re: [linus:master] [jbd2] 6a3afb6ac6: fileio.latency_95th_ms 92.5% regression

[Jan Kara]

Hello!

On Wed 03-01-24 11:31:39, Zhang Yi wrote:
> On 2024/1/2 15:31, kernel test robot wrote:
> > 
> > 
> > Hello,
> > 
> > kernel test robot noticed a 92.5% regression of fileio.latency_95th_ms on:
> 
> This seems a little weird, the tests doesn't use blk-cgroup, and the patch
> increase IO priority in WBT, so there shouldn't be any negative influence in
> theory.

I don't have a great explanation either but there could be some impact e.g.
due to a different request merging of IO done by JBD2 vs the flush worker or
something like that. Note that the throughput reduction is only 5.7% so it
is not huge.

> I've tested sysbench on my machine with Intel Xeon Gold 6240 CPU,
> 400GB memory with HDD disk, and couldn't reproduce this regression.
> 
> ==
> Without 6a3afb6ac6 ("jbd2: increase the journal IO's priority")
> ==
> 
>  $ sysbench fileio --events=0 --threads=128 --time=600 --file-test-mode=seqwr --file-total-size=68719476736 --file-io-mode=sync --file-num=1024 run
> 
>   sysbench 1.1.0-df89d34 (using bundled LuaJIT 2.1.0-beta3)
> 
>   Running the test with following options:
>   Number of threads: 128
>   Initializing random number generator from current time
> 
> 
>   Extra file open flags: (none)
>   1024 files, 64MiB each
>   64GiB total file size
>   Block size 16KiB
>   Periodic FSYNC enabled, calling fsync() each 100 requests.
>   Calling fsync() at the end of test, Enabled.
>   Using synchronous I/O mode
>   Doing sequential write (creation) test
>   Initializing worker threads...
> 
>   Threads started!
> 
> 
>   Throughput:
>            read:  IOPS=0.00 0.00 MiB/s (0.00 MB/s)
>            write: IOPS=31961.19 499.39 MiB/s (523.65 MB/s)
>            fsync: IOPS=327500.24

Well, your setup seems to be very different from what LKP was using. You
are achieving ~500 MB/s (likely because all the files fit into the cache
and more or less even within the dirty limit of the page cache) while LKP
run achieves only ~54 MB/s (i.e., we are pretty much bound by the rather
slow disk). I'd try running with something like 32GB of RAM to really see
the disk speed impact...

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR