On Thu, Apr 8, 2010 at 10:09 AM, Jens Axboe <jens.axboe@xxxxxxxxxx> wrote: > On Thu, Apr 08 2010, Vivek Goyal wrote: >> On Thu, Apr 08, 2010 at 01:04:42PM +0200, Jens Axboe wrote: >> > On Wed, Apr 07 2010, Vivek Goyal wrote: >> > > On Wed, Apr 07, 2010 at 05:18:12PM -0400, 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. >> > > > ... >> > > So we got to take care of two issues now. >> > > >> > > - Make it work with dm/md devices also. Somehow shall have to propogate >> > > this yield semantic down the stack. >> > >> > The way that Jeff set it up, it's completely parallel to eg congestion >> > or unplugging. So that should be easily doable. >> > >> >> Ok, so various dm targets now need to define "yield_fn" and propogate the >> yield call to all the component devices. > > Exactly. To do so doesn't DM (and MD) need a blk_queue_yield() setter to establish its own yield_fn? The established dm_yield_fn would call blk_yield() for all real devices in a given DM target. Something like how blk_queue_merge_bvec() or blk_queue_make_request() allow DM to provide functional extensions. I'm not seeing such a yield_fn hook for stacking drivers to use. And as is, jbd and jbd2 just call blk_yield() directly and there is no way for the block layer to call into DM. What am I missing? Thanks, Mike -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
