Mike Snitzer <snitzer@xxxxxxxxxx> writes: > 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? Nothing, it is I who am missing something (extra code). When I send out the next version, I'll add the setter function and ensure that queue->yield_fn is called from blk_yield. Hopefully that's not viewed as upside down. We'll see. Thanks for the review, Mike! -Jeff -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
