Date: Fri, 11 Sep 2015 20:00:39 +0300 From: Alexander Motin <mav@FreeBSD.org> To: Matthew Ahrens <mahrens@delphix.com>, freebsd-fs <freebsd-fs@freebsd.org> Subject: Re: zfs_trim_enabled destroys zio_free() performance Message-ID: <55F308B7.3020302@FreeBSD.org> In-Reply-To: <CAJjvXiE2mRT4=kPMk3gwiT-3ykeAhaYBx6Tw6HgXhs2=XZWWFg@mail.gmail.com> References: <CAJjvXiE2mRT4=kPMk3gwiT-3ykeAhaYBx6Tw6HgXhs2=XZWWFg@mail.gmail.com>
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Hi. The code in question was added by me at r253992. Commit message tells it was made to decouple locks. I don't remember much more details, but may be it can be redone somehow else. On 11.09.2015 19:07, Matthew Ahrens wrote: > I discovered that when destroying a ZFS snapshot, we can end up using > several seconds of CPU via this stack trace: > > kernel`spinlock_exit+0x2d > kernel`taskqueue_enqueue+0x12c > zfs.ko`zio_issue_async+0x7c > zfs.ko`zio_execute+0x162 > zfs.ko`dsl_scan_free_block_cb+0x15f > zfs.ko`bpobj_iterate_impl+0x25d > zfs.ko`bpobj_iterate_impl+0x46e > zfs.ko`dsl_scan_sync+0x152 > zfs.ko`spa_sync+0x5c1 > zfs.ko`txg_sync_thread+0x3a6 > kernel`fork_exit+0x9a > kernel`0xffffffff80d0acbe > 6558 ms > > This is not good for performance since, in addition to the CPU cost, it > doesn't allow the sync thread to do anything else, and this is > observable as periods where we don't do any write i/o to disk for > several seconds. > > The problem is that when zfs_trim_enabled is set (which it is by > default), zio_free_sync() always sets ZIO_STAGE_ISSUE_ASYNC, causing the > free to be dispatched to a taskq. Since each task completes very > quickly, there is a large locking and context switching overhead -- we > would be better off just processing the free in the caller's context. > > I'm not sure exactly why we need to go async when trim is enabled, but > it seems like at least we should not bother going async if trim is not > actually being used (e.g. with an all-spinning-disk pool). It would > also be worth investigating not going async even when trim is useful > (e.g. on SSD-based pools). > > Here is the relevant code: > > zio_free_sync(): > if (zfs_trim_enabled) > stage |= ZIO_STAGE_ISSUE_ASYNC | ZIO_STAGE_VDEV_IO_START | > ZIO_STAGE_VDEV_IO_ASSESS; > /* > * GANG and DEDUP blocks can induce a read (for the gang block > header, > * or the DDT), so issue them asynchronously so that this thread is > * not tied up. > */ > else if (BP_IS_GANG(bp) || BP_GET_DEDUP(bp)) > stage |= ZIO_STAGE_ISSUE_ASYNC; > > --matt -- Alexander Motin
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