From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Snitzer Subject: Re: dm: reduce the number of processes per dm device Date: Thu, 8 Oct 2015 16:15:30 -0400 Message-ID: <20151008201530.GA12983@redhat.com> References: <20151008190817.GA12788@redhat.com> Reply-To: device-mapper development Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com To: Mikulas Patocka Cc: dm-devel@redhat.com, "Alasdair G. Kergon" , Zdenek Kabelac List-Id: dm-devel.ids On Thu, Oct 08 2015 at 3:59pm -0400, Mikulas Patocka wrote: > > > On Thu, 8 Oct 2015, Mike Snitzer wrote: > > > On Thu, Oct 08 2015 at 12:15pm -0400, > > Mikulas Patocka 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?