On 31.05.23 10:57, Ojaswin Mujoo wrote: > On Tue, May 30, 2023 at 06:28:22PM +0200, Sedat Dilek wrote: >> On Tue, May 30, 2023 at 3:25 PM Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx> wrote: >>> >>> This reverts commit 32c0869370194ae5ac9f9f501953ef693040f6a1. >>> >>> The reverted commit was intended to remove a dead check however it was observed >>> that this check was actually being used to exit early instead of looping >>> sbi->s_mb_max_to_scan times when we are able to find a free extent bigger than >>> the goal extent. Due to this, a my performance tests (fsmark, parallel file >>> writes in a highly fragmented FS) were seeing a 2x-3x regression. >>> >>> Example, the default value of the following variables is: >>> >>> sbi->s_mb_max_to_scan = 200 >>> sbi->s_mb_min_to_scan = 10 >>> >>> In ext4_mb_check_limits() if we find an extent smaller than goal, then we return >>> early and try again. This loop will go on until we have processed >>> sbi->s_mb_max_to_scan(=200) number of free extents at which point we exit and >>> just use whatever we have even if it is smaller than goal extent. >>> >>> Now, the regression comes when we find an extent bigger than goal. Earlier, in >>> this case we would loop only sbi->s_mb_min_to_scan(=10) times and then just use >>> the bigger extent. However with commit 32c08693 that check was removed and hence >>> we would loop sbi->s_mb_max_to_scan(=200) times even though we have a big enough >>> free extent to satisfy the request. The only time we would exit early would be >>> when the free extent is *exactly* the size of our goal, which is pretty uncommon >>> occurrence and so we would almost always end up looping 200 times. >>> >>> Hence, revert the commit by adding the check back to fix the regression. Also >>> add a comment to outline this policy. >> >> I applied this single patch of your series v2 on top of Linux v6.4-rc4. >> >> So, if this is a regression I ask myself if this is material for Linux 6.4? >> >> Can you comment on this, please? > > Since this patch fixes a regression I think it should ideally go in > Linux 6.4 Ted can speak up for himself, but maybe this might speed things up: A lot of maintainers in a case like this want fixes (like this) submitted separately from other changes (like the rest of this series). /me hopes this will help and not confuse anything Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat) -- Everything you wanna know about Linux kernel regression tracking: https://linux-regtracking.leemhuis.info/about/#tldr If I did something stupid, please tell me, as explained on that page.
