Messages in this thread | | | Date | Thu, 17 Sep 2015 19:56:47 -0400 | From | Chris Mason <> | Subject | Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() |
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On Thu, Sep 17, 2015 at 04:08:19PM -0700, Linus Torvalds wrote: > On Thu, Sep 17, 2015 at 3:42 PM, Chris Mason <clm@fb.com> wrote: > > > > Playing around with the plug a little, most of the unplugs are coming > > from the cond_resched_lock(). Not really sure why we are doing the > > cond_resched() there, we should be doing it before we retake the lock > > instead. > > > > This patch takes my box (with dirty thresholds at 1.5GB/3GB) from 195K > > files/sec up to 213K. Average IO size is the same as 4.3-rc1. > > Ok, so at least for you, part of the problem really ends up being that > there's a mix of the "synchronous" unplugging (by the actual explicit > "blk_finish_plug(&plug);") and the writeback that is handed off to > kblockd_workqueue. > > I'm not seeing why that should be an issue. Sure, there's some CPU > overhead to context switching, but I don't see that it should be that > big of a deal. > > I wonder if there is something more serious wrong with the kblockd_workqueue.
I'm driving the box pretty hard, it's right on the line between CPU bound and IO bound. So I've got 32 fs_mark processes banging away and 32 CPUs (16 really, with hyperthreading).
They are popping in and out of balance_dirty_pages() so I have high CPU utilization alternating with high IO wait times. There no reads at all, so all of these waits are for buffered writes.
People in balance_dirty_pages are indirectly waiting on the unplug, so maybe the context switch overhead on a loaded box is enough to explain it. We've definitely gotten more than 9% by inlining small synchronous items in btrfs in the past, but those were more explicitly synchronous.
I know it's painfully hand wavy. I don't see any other users of the kblockd workqueues, and the perf profiles don't jump out at me. I'll feel better about the patch if Dave confirms any gains.
-chris
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