On Thu, Mar 10, 2022 at 2:10 PM Song Liu <song@xxxxxxxxxx> wrote:
>
> On Wed, Mar 9, 2022 at 11:23 PM Song Liu <song@xxxxxxxxxx> wrote:
> >
> > On Wed, Mar 9, 2022 at 10:48 PM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> > >
> > > On Tue, Mar 08, 2022 at 10:42:09PM -0800, Song Liu wrote:
> > > > RAID arrays check/repair operations benefit a lot from merging requests.
> > > > If we only check the previous entry for merge attempt, many merge will be
> > > > missed. As a result, significant regression is observed for RAID check
> > > > and repair.
> > > >
> > > > Fix this by checking more than just the previous entry when
> > > > plug->multiple_queues == true.
> > >
> > > But this also means really significant CPU overhead for all other
> > > workloads.
> >
> > Would the following check help with these workloads?
> >
> > if (!plug->multiple_queues)
> > break;
> >
> > >
> > > >
> > > > This improves the check/repair speed of a 20-HDD raid6 from 19 MB/s to
> > > > 103 MB/s.
> > >
> > > What driver uses multiple queues for HDDs?
> > >
> > > Can you explain the workload submitted by a md a bit better? I wonder
> > > if we can easily do the right thing straight in the md driver.
> >
> > It is the md sync_thread doing check and repair. Basically, the md
> > thread reads all
> > the disks and computes parity from data.
> >
> > Maybe we should add a new flag to struct blk_plug for this special case?
>
> I meant something like:
>
> diff --git c/block/blk-core.c w/block/blk-core.c
> index 1039515c99d6..4fb09243e908 100644
> --- c/block/blk-core.c
> +++ w/block/blk-core.c
> @@ -1303,6 +1303,12 @@ void blk_finish_plug(struct blk_plug *plug)
> }
> EXPORT_SYMBOL(blk_finish_plug);
>
> +void blk_plug_merge_aggressively(struct blk_plug *plug)
> +{
> + plug->aggresive_merge = true;
> +}
> +EXPORT_SYMBOL(blk_plug_merge_aggressively);
> +
> void blk_io_schedule(void)
> {
> /* Prevent hang_check timer from firing at us during very long I/O */
Missed one change:
--- c/block/blk-core.c
+++ w/block/blk-core.c
@@ -1188,6 +1188,7 @@ void blk_start_plug_nr_ios(struct blk_plug
*plug, unsigned short nr_ios)
plug->multiple_queues = false;
plug->has_elevator = false;
plug->nowait = false;
+ plug->aggresive_merge = false;
INIT_LIST_HEAD(&plug->cb_list);
/*
