Messages in this thread | | | From | Jeff Moyer <> | Subject | Re: [PATCH 0/6 v6][RFC] jbd[2]: enhance fsync performance when using CFQ | Date | Fri, 02 Jul 2010 16:32:31 -0400 |
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Vivek Goyal <vgoyal@redhat.com> writes:
> On Fri, Jul 02, 2010 at 03:58:13PM -0400, Jeff Moyer wrote: > > [..] >> Changes from the last posting: >> - Yielding no longer expires the current queue. Instead, it sets up new >> requests from the target process so that they are issued in the yielding >> process' cfqq. This means that we don't need to worry about losing group >> or workload share. >> - Journal commits are now synchronous I/Os, which was required to get any >> sort of performance out of the fs_mark process in the presence of a >> competing reader. >> - WRITE_SYNC I/O no longer sets RQ_NOIDLE, for a similar reason. > > Hi Jeff, > > So this patchset relies on idling on WRITE_SYNC queues. Though in general > we don't have examples that why one should idle on processes doing WRITE_SYNC > IO because previous IO does not tell anything about the upcoming IO. I am > bringing up this point again to make sure that fundamentally we agree that > continue to idle on WRITE_SYNC is the right thing to do otherwise this patch > will fall apart.
I think a mail server would be an example of an application that might do this. I'll see if I can get a real world test case (or perhaps some real world data) and verify that.
I agree that if we choose not to idle on write's, then this approach can be thrown out the window.
> I have yet to go through the patch in detail but allowing other queue to > dispatch requests in the same queue sounds like queue merging. So can > we use that semantics to say elv_merge_context() or elv_merge_queue() > instead of elv_yield(). In the code we can just merge the two queues when > the next request comes in and separate them out at the slice expiry I > guess.
I considered that approach, but then you run into all of the questions about losing fairness across workloads and across groups. I believe the approach I've taken here is *significantly* simpler than merging and unmerging would be.
Cheers, Jeff
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