Messages in this thread |  | | Subject | Re: [PATCH v2 00/12] Improve Raid5 Lock Contention | From | Guoqing Jiang <> | Date | Sun, 24 Apr 2022 15:53:54 +0800 |
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On 4/21/22 3:54 AM, Logan Gunthorpe wrote: > Hi, > > This is v2 of this series which addresses Christoph's feedback and > fixes some bugs. The first posting is at [1]. A git branch is > available at [2]. > > -- > > I've been doing some work trying to improve the bulk write performance > of raid5 on large systems with fast NVMe drives. The bottleneck appears > largely to be lock contention on the hash_lock and device_lock. This > series improves the situation slightly by addressing a couple of low > hanging fruit ways to take the lock fewer times in the request path. > > Patch 9 adjusts how batching works by keeping a reference to the > previous stripe_head in raid5_make_request(). Under most situtations, > this removes the need to take the hash_lock in stripe_add_to_batch_list() > which should reduce the number of times the lock is taken by a factor of > about 2. > > Patch 12 pivots the way raid5_make_request() works. Before the patch, the > code must find the stripe_head for every 4KB page in the request, so each > stripe head must be found once for every data disk. The patch changes this > so that all the data disks can be added to a stripe_head at once and the > number of times the stripe_head must be found (and thus the number of > times the hash_lock is taken) should be reduced by a factor roughly equal > to the number of data disks. > > The remaining patches are just cleanup and prep patches for those two > patches. > > Doing apples to apples testing this series on a small VM with 5 ram > disks, I saw a bandwidth increase of roughly 14% and lock contentions > on the hash_lock (as reported by lock stat) reduced by more than a factor > of 5 (though it is still significantly contended). > > Testing on larger systems with NVMe drives saw similar small bandwidth > increases from 3% to 20% depending on the parameters. Oddly small arrays > had larger gains, likely due to them having lower starting bandwidths; I > would have expected larger gains with larger arrays (seeing there > should have been even fewer locks taken in raid5_make_request()). > > Logan > > [1] https://lkml.kernel.org/r/20220407164511.8472-1-logang@deltatee.com > [2] https://github.com/sbates130272/linux-p2pmem raid5_lock_cont_v2 > > -- > > Changes since v1: > - Rebased on current md-next branch (190a901246c69d79) > - Added patch to create a helper for checking if a sector > is ahead of the reshape (per Christoph) > - Reworked the __find_stripe() patch to create a find_get_stripe() > helper (per Christoph) > - Added more patches to further refactor raid5_make_request() and > pull most of the loop body into a helper function (per Christoph) > - A few other minor cleanups (boolean return, droping casting when > printing sectors, commit message grammar) as suggested by Christoph. > - Fixed two uncommon but bad data corruption bugs in that were found. > > -- > > Logan Gunthorpe (12): > md/raid5: Factor out ahead_of_reshape() function > md/raid5: Refactor raid5_make_request loop > md/raid5: Move stripe_add_to_batch_list() call out of add_stripe_bio() > md/raid5: Move common stripe count increment code into __find_stripe() > md/raid5: Factor out helper from raid5_make_request() loop > md/raid5: Drop the do_prepare flag in raid5_make_request() > md/raid5: Move read_seqcount_begin() into make_stripe_request() > md/raid5: Refactor for loop in raid5_make_request() into while loop > md/raid5: Keep a reference to last stripe_head for batch > md/raid5: Refactor add_stripe_bio() > md/raid5: Check all disks in a stripe_head for reshape progress > md/raid5: Pivot raid5_make_request()
Generally, I don't object the cleanup patches since the code looks more cleaner. But my concern is that since some additional function calls are added to hot path (raid5_make_request), could the performance be affected?
And I think patch 9 and patch 12 are helpful for performance improvement, did you measure the performance without those cleanup patches?
Thanks, Guoqing
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