Re: [PATCH v5 00/11] simplify block layer based on immutable biovecs

[Mike Snitzer]

On Mon, Jul 13 2015 at  1:12am -0400,
Ming Lin <mlin@xxxxxxxxxx> wrote:

> On Mon, 2015-07-06 at 00:11 -0700, mlin@xxxxxxxxxx wrote:
> > Hi Mike,
> > 
> > On Wed, 2015-06-10 at 17:46 -0400, Mike Snitzer wrote:
> > > I've been busy getting DM changes for the 4.2 merge window finalized.
> > > As such I haven't connected with others on the team to discuss this
> > > issue.
> > > 
> > > I'll see if we can make time in the next 2 days.  But I also have
> > > RHEL-specific kernel deadlines I'm coming up against.
> > > 
> > > Seems late to be staging this extensive a change for 4.2... are you
> > > pushing for this code to land in the 4.2 merge window?  Or do we have
> > > time to work this further and target the 4.3 merge?
> > > 
> > 
> > 4.2-rc1 was out.
> > Would you have time to work together for 4.3 merge? 
> 
> Ping ...
> 
> What can I do to move forward?

You can show further testing.  Particularly that you've covered all the
edge cases.

Until someone can produce some perf test results where they are actually
properly controlling for the splitting, we have no useful information.

The primary concerns associated with this patchset are:
1) In the context of RAID, XFS's use of bio_add_page() used to build up
   optimal IOs when the underlying block device provides striping info
   via IO limits.  With this patchset how large will bios become in
   practice _without_ bio_add_page() being bounded by the underlying IO
   limits?

2) The late splitting that occurs for the (presummably) large bios that
   are sent down.. how does it cope/perform in the face of very
   low/fragmented system memory?

3) More open-ended comment than question: Linux has evolved to perform
   well on "enterprise" systems.  We generally don't fall off a cliff on 
   performance like we used to.  The concern associated with this
   patchset is that if it goes in without _real_ due-diligence on
   "enterprise" scale systems and workloads it'll be too late once we
   notice the problem(s).

So we really need answers to 1 and 2 above in order to feel better about
the risks associated 3.

Alasdair's feedback to you on testing still applies (and hasn't been
done AFAIK):
https://www.redhat.com/archives/dm-devel/2015-May/msg00203.html

Particularly:
"you might need to instrument the kernels to tell you the sizes of the
bios being created and the amount of splitting actually happening."

and

"You may also want to test systems with a restricted amount of available
memory to show how the splitting via worker thread performs.  (Again,
instrument to prove the extent to which the new code is being exercised.)"

> This patchset not only simplify block layer a lot, it's also a
> prerequisite of the direct IO rewrite patches, which I saw 40%
> performance improvement for null_blk and 10% improvement for NVMe
> drives. I have been fixing bugs for the direct IO patches. I'll post it
> once it passes xfstests.
> 
> Mike,
> Can I have your ACK? Or do you have other test plan?

I'm not the only person with concerns.  I share Alasdair's concerns.
Jeff Moyer is also concerned about the implications of this patchset.
We're all in favor of this patchset's cleanup _if and only if_ it can be
proven that we aren't going to be falling off a cliff on performance due
to some pathological workload (be it under memory pressure or whatever).

Apologies for not being able to put time to this like I hoped.  But that
doesn't mean you are off the hook on showing you've done the testing and
understand the scope and implications of the changes you're pushing for.

I will do additional review to answer 1 and 2 above.  And Jeff Moyer
told me he'd test the patchset on one of his testbeds.

But if you can help answer 1 and 2 above that'd go a long way.

Thanks,
Mike
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