Re: dm: reduce the number of processes per dm device

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Oct 08 2015 at  3:59pm -0400,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> 
> 
> On Thu, 8 Oct 2015, Mike Snitzer wrote:
> 
> > On Thu, Oct 08 2015 at 12:15pm -0400,
> > Mikulas Patocka <mpatocka@redhat.com> 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?