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    SubjectRe: [patch,rfc v2] ext3/4: enhance fsync performance when using cfq
    On Wed, Apr 07 2010, Jeff Moyer wrote:
    > Hi again,
    >
    > So, here's another stab at fixing this. This patch is very much an RFC,
    > so do not pull it into anything bound for Linus. ;-) For those new to
    > this topic, here is the original posting: http://lkml.org/lkml/2010/4/1/344
    >
    > The basic problem is that, when running iozone on smallish files (up to
    > 8MB in size) and including fsync in the timings, deadline outperforms
    > CFQ by a factor of about 5 for 64KB files, and by about 10% for 8MB
    > files. From examining the blktrace data, it appears that iozone will
    > issue an fsync() call, and will have to wait until it's CFQ timeslice
    > has expired before the journal thread can run to actually commit data to
    > disk.
    >
    > The approach below puts an explicit call into the filesystem-specific
    > fsync code to yield the disk so that the jbd[2] process has a chance to
    > issue I/O. This bring performance of CFQ in line with deadline.
    >
    > There is one outstanding issue with the patch that Vivek pointed out.
    > Basically, this could starve out the sync-noidle workload if there is a
    > lot of fsync-ing going on. I'll address that in a follow-on patch. For
    > now, I wanted to get the idea out there for others to comment on.
    >
    > Thanks a ton to Vivek for spotting the problem with the initial
    > approach, and for his continued review.

    I like the concept, it's definitely useful (and your results amply
    demonstrate that). I was thinking if there was a way in through the ioc
    itself, rather than bdi -> queue and like you are doing. But I can't
    think of a nice way to do it, so this is probably as good as it gets.

    > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
    > index dee9d93..a76ccd1 100644
    > --- a/block/cfq-iosched.c
    > +++ b/block/cfq-iosched.c
    > @@ -2065,7 +2065,7 @@ static void cfq_choose_cfqg(struct cfq_data *cfqd)
    > cfqd->serving_group = cfqg;
    >
    > /* Restore the workload type data */
    > - if (cfqg->saved_workload_slice) {
    > + if (cfqg && cfqg->saved_workload_slice) {
    > cfqd->workload_expires = jiffies + cfqg->saved_workload_slice;
    > cfqd->serving_type = cfqg->saved_workload;
    > cfqd->serving_prio = cfqg->saved_serving_prio;

    Unrelated change?

    > +static void cfq_yield(struct request_queue *q)
    > +{
    > + struct cfq_data *cfqd = q->elevator->elevator_data;
    > + struct cfq_io_context *cic;
    > + struct cfq_queue *cfqq;
    > + unsigned long flags;
    > +
    > + cic = cfq_cic_lookup(cfqd, current->io_context);
    > + if (!cic)
    > + return;
    > +
    > + spin_lock_irqsave(q->queue_lock, flags);

    spin_lock_irq() is sufficient here.

    --
    Jens Axboe



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