On Thu, 8 Oct 2015, Mike Snitzer wrote:
> On Thu, Oct 08 2015 at 3:59pm -0400,
> Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
>
> >
> >
> > On Thu, 8 Oct 2015, Mike Snitzer wrote:
> >
> > > On Thu, Oct 08 2015 at 12:15pm -0400,
> > > Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
> > >
> > > > The patch 54efd50bfd873e2dbf784e0b21a8027ba4299a3e ("block: make
> > > > generic_make_request handle arbitrarily sized bios") makes it possible for
> > > > block devices to process large bios. The patch allocates a new bio set
> > > > queue->bio_split for each device, this bio set is used for allocating bios
> > > > when the driver needs to split large bios.
> > > >
> > > > Each bio_set allocates a workqueue process, thus the above patch increases
> > > > the number of processes allocated per block device.
> > > >
> > > > Device mapper doesn't need the queue->bio_split bio_set, thus we can
> > > > deallocate it. This reduces the number of allocated processes per
> > > > dm-device from 3 to 2.
> > >
> > > This header needs more context added, specifically we need to tell the
> > > reader the answer to: why doesn't DM need queue->bio_split?
> >
> > Dm doesn't need queue->bio_split because it has its own bioset md->bs. We
> > can't use queue->bio_split instead of md->bs because md->bs has non-zero
> > front pad depending on targets loaded in the table.
>
> Sure but my point was that bio-based DM targets by definition aren't the
> last device in a stack.
>
> > > Is this a resource that only the lowest layer's request_queue would
> > > need? And given DM's stacking nature it doesn't need it simply because
> > > it'll never be the lowest layer?
> >
> > All request-based drivers need queue->bio_split, but some non-dm bio-based
> > drivers need it too.
>
> Not all request-based drivers right? request-based DM (aka DM mpath)
> shouldn't _need_ queue->bio_split either right?
It uses queue->bio_split because it is used at the begining of
blk_queue_bio. All request-based drivers use blk_queue_bio.
Mikulas
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