Messages in this thread | | | Date | Mon, 19 Jul 2010 16:31:09 -0400 | From | Vivek Goyal <> | Subject | Re: [PATCH 0/2] cfq-iosched: fixing RQ_NOIDLE handling. |
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On Mon, Jul 19, 2010 at 12:08:23PM -0400, Jeff Moyer wrote: > Vivek Goyal <vgoyal@redhat.com> writes: > > > On Tue, Jul 13, 2010 at 04:30:23PM -0400, Jeff Moyer wrote: > >> Vivek Goyal <vgoyal@redhat.com> writes: > > > I don't mind looking at traces. Do let me know where can I access those. > > Forwarded privately. > > >> Now, to answer your question, the jbd2 thread runs and issues a barrier, > >> which causes a forced dispatch of requests. After that a new queue is > >> selected, and since the fs_mark thread is blocked on the journal commit, > >> it's always the fio process that gets to run. > > > > Ok, that explains it. So somehow after the barrier, fio always wins > > as issues next read request before the fs_mark is able to issue the > > next set of writes. > > > >> > >> This, of course, raises the question of why the blk_yield patches didn't > >> run into the same problem. Looking back at some saved traces, I don't > >> see WBS (write barrier sync) requests, so I wonder if barriers weren't > >> supported by my last storage system. > > > > I think that blk_yield patches will also run into the same issue if > > barriers are enabled. > > Agreed. > > Here are the results again with barriers disabled for Corrado's patch: > > fs_mark: 348.2 files/sec > fio: 53324.6 KB/s > > Remember that deadline was seeing 450 files/sec and 78 MB/s. So, in > this case, the buffered reader appears to be starved. Looking into this > further, I found that the journal thread is running with I/O priority 0, > while the fio and fs_mark processes are running at the default (4). > Because the jbd thread has a higher I/O priority, its requests are > always closer to the front of the sort list, and thus the sync-noidle > workload is chosen more often than the sync workload. This essentially > results in an elevated I/O priority for the fs_mark process as well. > While troubling, that problem is not directly related to the problem > we're looking at. > > So, I'm still in favor of Corrado's approach. Are there any remaining > dissenting opinions on this?
Nope. I am fine with moving all WRITE_SYNC with RQ_NOIDLE to sync-noidle tree and also marking jbd writes as WRITE_SYNC. By bringing dependent threads on single service tree, we don't have to worry about slice yielding.
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Thanks Vivek
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