From: Ming Lei <ming.lei@redhat.com> To: Mike Snitzer <snitzer@redhat.com> Cc: axboe@kernel.dk, martin.petersen@oracle.com, linux-block@vger.kernel.org, dm-devel@redhat.com, jdorminy@redhat.com, bjohnsto@redhat.com Subject: Re: [PATCH v2] block: use gcd() to fix chunk_sectors limit stacking Date: Thu, 3 Dec 2020 11:26:08 +0800 [thread overview] Message-ID: <20201203032608.GD540033@T590> (raw) In-Reply-To: <20201201160709.31748-1-snitzer@redhat.com> On Tue, Dec 01, 2020 at 11:07:09AM -0500, Mike Snitzer wrote: > commit 22ada802ede8 ("block: use lcm_not_zero() when stacking > chunk_sectors") broke chunk_sectors limit stacking. chunk_sectors must > reflect the most limited of all devices in the IO stack. > > Otherwise malformed IO may result. E.g.: prior to this fix, > ->chunk_sectors = lcm_not_zero(8, 128) would result in > blk_max_size_offset() splitting IO at 128 sectors rather than the > required more restrictive 8 sectors. What is the user-visible result of splitting IO at 128 sectors? I understand it isn't related with correctness, because the underlying queue can split by its own chunk_sectors limit further. So is the issue too many further-splitting on queue with chunk_sectors 8? then CPU utilization is increased? Or other issue? > > And since commit 07d098e6bbad ("block: allow 'chunk_sectors' to be > non-power-of-2") care must be taken to properly stack chunk_sectors to > be compatible with the possibility that a non-power-of-2 chunk_sectors > may be stacked. This is why gcd() is used instead of reverting back > to using min_not_zero(). I guess gcd() won't be better because gcd(a,b) is <= max(a, b), so bio size is decreased much with gcd(a, b), and IO performance should be affected. Maybe worse than min_not_zero(a, b) which is often > gcd(a, b). Thanks, Ming
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From: Ming Lei <ming.lei@redhat.com> To: Mike Snitzer <snitzer@redhat.com> Cc: axboe@kernel.dk, martin.petersen@oracle.com, jdorminy@redhat.com, bjohnsto@redhat.com, linux-block@vger.kernel.org, dm-devel@redhat.com Subject: Re: [dm-devel] [PATCH v2] block: use gcd() to fix chunk_sectors limit stacking Date: Thu, 3 Dec 2020 11:26:08 +0800 [thread overview] Message-ID: <20201203032608.GD540033@T590> (raw) In-Reply-To: <20201201160709.31748-1-snitzer@redhat.com> On Tue, Dec 01, 2020 at 11:07:09AM -0500, Mike Snitzer wrote: > commit 22ada802ede8 ("block: use lcm_not_zero() when stacking > chunk_sectors") broke chunk_sectors limit stacking. chunk_sectors must > reflect the most limited of all devices in the IO stack. > > Otherwise malformed IO may result. E.g.: prior to this fix, > ->chunk_sectors = lcm_not_zero(8, 128) would result in > blk_max_size_offset() splitting IO at 128 sectors rather than the > required more restrictive 8 sectors. What is the user-visible result of splitting IO at 128 sectors? I understand it isn't related with correctness, because the underlying queue can split by its own chunk_sectors limit further. So is the issue too many further-splitting on queue with chunk_sectors 8? then CPU utilization is increased? Or other issue? > > And since commit 07d098e6bbad ("block: allow 'chunk_sectors' to be > non-power-of-2") care must be taken to properly stack chunk_sectors to > be compatible with the possibility that a non-power-of-2 chunk_sectors > may be stacked. This is why gcd() is used instead of reverting back > to using min_not_zero(). I guess gcd() won't be better because gcd(a,b) is <= max(a, b), so bio size is decreased much with gcd(a, b), and IO performance should be affected. Maybe worse than min_not_zero(a, b) which is often > gcd(a, b). Thanks, Ming -- dm-devel mailing list dm-devel@redhat.com https://www.redhat.com/mailman/listinfo/dm-devel
next prev parent reply other threads:[~2020-12-03 3:28 UTC|newest] Thread overview: 66+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-11-30 17:18 [PATCH] block: revert to using min_not_zero() when stacking chunk_sectors Mike Snitzer 2020-11-30 17:18 ` [dm-devel] " Mike Snitzer 2020-11-30 20:51 ` John Dorminy 2020-11-30 20:51 ` [dm-devel] " John Dorminy 2020-11-30 23:24 ` Mike Snitzer 2020-11-30 23:24 ` [dm-devel] " Mike Snitzer 2020-12-01 0:21 ` John Dorminy 2020-12-01 0:21 ` [dm-devel] " John Dorminy 2020-12-01 2:12 ` Mike Snitzer 2020-12-01 2:12 ` [dm-devel] " Mike Snitzer 2020-12-01 16:07 ` [PATCH v2] block: use gcd() to fix chunk_sectors limit stacking Mike Snitzer 2020-12-01 16:07 ` [dm-devel] " Mike Snitzer 2020-12-01 17:43 ` John Dorminy 2020-12-01 17:43 ` [dm-devel] " John Dorminy 2020-12-01 17:53 ` Jens Axboe 2020-12-01 17:53 ` [dm-devel] " Jens Axboe 2020-12-01 18:02 ` Martin K. Petersen 2020-12-01 18:02 ` [dm-devel] " Martin K. Petersen 2020-12-02 3:38 ` [PATCH] dm: " Jeffle Xu 2020-12-02 3:38 ` [dm-devel] " Jeffle Xu 2020-12-02 3:38 ` Jeffle Xu 2020-12-02 3:38 ` [dm-devel] " Jeffle Xu 2020-12-02 3:57 ` JeffleXu 2020-12-02 3:57 ` [dm-devel] " JeffleXu 2020-12-02 5:03 ` Mike Snitzer 2020-12-02 5:03 ` [dm-devel] " Mike Snitzer 2020-12-02 5:14 ` Mike Snitzer 2020-12-02 5:14 ` [dm-devel] " Mike Snitzer 2020-12-02 6:31 ` JeffleXu 2020-12-02 6:31 ` [dm-devel] " JeffleXu 2020-12-02 6:35 ` JeffleXu 2020-12-02 6:35 ` [dm-devel] " JeffleXu 2020-12-02 6:28 ` JeffleXu 2020-12-02 6:28 ` [dm-devel] " JeffleXu 2020-12-02 7:10 ` JeffleXu 2020-12-02 7:10 ` [dm-devel] " JeffleXu 2020-12-02 15:11 ` Mike Snitzer 2020-12-02 15:11 ` [dm-devel] " Mike Snitzer 2020-12-03 1:48 ` JeffleXu 2020-12-03 1:48 ` JeffleXu 2020-12-03 3:26 ` Ming Lei [this message] 2020-12-03 3:26 ` [dm-devel] [PATCH v2] block: " Ming Lei 2020-12-03 14:33 ` Mike Snitzer 2020-12-03 14:33 ` [dm-devel] " Mike Snitzer 2020-12-03 16:27 ` Keith Busch 2020-12-03 16:27 ` [dm-devel] " Keith Busch 2020-12-03 17:56 ` Mike Snitzer 2020-12-03 17:56 ` [dm-devel] " Mike Snitzer 2020-12-04 1:45 ` Ming Lei 2020-12-04 1:45 ` [dm-devel] " Ming Lei 2020-12-04 2:11 ` Mike Snitzer 2020-12-04 2:11 ` [dm-devel] " Mike Snitzer 2020-12-04 6:22 ` Damien Le Moal 2020-12-04 6:22 ` Damien Le Moal 2020-12-04 1:12 ` Ming Lei 2020-12-04 1:12 ` [dm-devel] " Ming Lei 2020-12-04 2:03 ` Mike Snitzer 2020-12-04 2:03 ` [dm-devel] " Mike Snitzer 2020-12-04 3:59 ` Ming Lei 2020-12-04 3:59 ` [dm-devel] " Ming Lei 2020-12-04 16:47 ` Mike Snitzer 2020-12-04 16:47 ` [dm-devel] " Mike Snitzer 2020-12-04 17:32 ` [RFC PATCH] dm: fix IO splitting [was: Re: [PATCH v2] block: use gcd() to fix chunk_sectors limit stacking] Mike Snitzer 2020-12-04 17:32 ` [dm-devel] " Mike Snitzer 2020-12-04 17:49 ` Mike Snitzer 2020-12-04 17:49 ` [dm-devel] " Mike Snitzer
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