Re: [RFC] Slow down of LTP tests aiodio_sparse.c and dio_sparse.c in kernel 6.6

[Jens Axboe <axboe@kernel.dk>]

On 7/19/24 2:13 PM, Petr Vorel wrote:
> Hi Jens, all,
> 
>> On 7/19/24 11:43 AM, Petr Vorel wrote:
>>> Hi all,
> 
>>> LTP AIO DIO tests aiodio_sparse.c [1] and dio_sparse.c [2] (using [3]) slowed
>>> down on kernel 6.6 on Btrfs and XFS, when run with default parameters. These
>>> tests create 100 MB sparse file and write zeros (using libaio or O_DIRECT) while
>>> 16 other processes reads the buffer and check only zero is there.
> 
>>> Runtime of this particular setup (i.e. 100 MB file) on Btrfs and XFS on the
>>> same system slowed down 9x (6.5: ~1 min 6.6: ~9 min). Ext4 is not affected.
>>> (Non default parameter creates much smaller file, thus the change is not that
>>> obvious).
> 
>>> Because the slowdown has been here for few kernel releases I suppose nobody
>>> complained and the test is somehow artificial (nobody uses this in a real world).
>>> But still it'd be good to double check the problem. I can bisect a particular
>>> commit.
> 
>>> Because 2 filesystems affected, could be "Improve asynchronous iomap DIO
>>> performance" [4] block layer change somehow related?
> 
>> No, because that got disabled before release for unrelated reasons. Why
>> don't you just bisect it, since you have a simple test case?
> 
> Jens, thanks for info. Sure, I'll bisect next week and report.
> 
> The reason I reported before bisecting is because it wouldn't be the
> first time the test was "artificial" and therefore reported problem
> was not fixed. If it's a real problem I would expect it would be also
> caught by other people or even by fstests.

Didn't look at the test cases, so yeah may very well be bogus. But it
also may not... And a bisection may help shine some light on that too,
outside of just highlighting what commit made it slower.

-- 
Jens Axboe