Re: [PATCH 1/2] blkdev: Submit discard bio in batches inblkdev_issue_discard()

[Lukas Czerner]

On Thu, 28 Apr 2011, Lukas Czerner wrote:

> On Wed, 27 Apr 2011, Jeff Moyer wrote:
> 
> > Lukas Czerner <lczerner@xxxxxxxxxx> writes:
> > 
> > > Currently we are waiting for every submitted REQ_DISCARD bio separately,
> > > but it can have unwanted consequences of repeatedly flushing the queue,
> > > so we rather submit bios in batches and wait for the entire batch, hence
> > > narrowing the window of other ios going in.
> > >
> > > Use bio_batch_end_io() and struct bio_batch for that purpose, the same
> > > is used by blkdev_issue_zeroout(). Also change bio_batch_end_io() so we
> > > always set !BIO_UPTODATE in the case of error and remove the check for
> > > bb, since we are the only user of this function and we always set this.
> > >
> > > Remove bio_get()/bio_put() from the blkdev_issue_discard() since
> > > bio_alloc() and bio_batch_end_io() is doing the same thing, hence it is
> > > not needed anymore.
> > >
> > > I have done simple dd testing with surprising results. The script I have
> > > used is:
> > >
> > > for i in $(seq 10); do
> > >         echo $i
> > >         dd if=/dev/sdb1 of=/dev/sdc1 bs=4k &
> > >         sleep 5
> > > done
> > > /usr/bin/time -f %e ./blkdiscard /dev/sdc1
> > >
> > > Running time of BLKDISCARD on the whole device:
> > > with patch              without patch
> > > 0.95                    15.58
> > >
> > > So we can see that in this artificial test the kernel with the patch
> > > applied is approx 16x faster in discarding the device.
> > 
> > I don't see any major problems here.  I would like you to test
> > explicitly for queue_flag_discard before submitting any bios, though.

Btw, this is already in there:

	if (!blk_queue_discard(q))
		return -EOPNOTSUPP;

at the top of the function.

> > Previously, after the first bio returned EOPNOTSUPP, you errored out.
> > Now, you submit all bios that will end with the same failure.  It's
> > definitely suboptimal, and you already have the q, so you might as well
> > test for this early on.
> > 
> > Also, as I noted to you on irc, another fix required in this area is
> > submitting discard requests (the final one in the batch) that do not
> > honor the discard granularity exported by the device.  That's not
> > directly related to this patch, though.
> > 
> > Cheers,
> > Jeff
> > 
> 
> Thanks for review and comments Jeff, I'll resend patches after I retest
> them.
> 
> -Lukas
> 

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