Re: [PATCH v12 00/20] DAX: Page cache bypass for filesystems on memory storage

From: Andrew Morton
Date: Tue Jan 06 2015 - 03:47:25 EST


On Mon, 5 Jan 2015 10:41:43 -0800 Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:

> On Wed, Dec 10, 2014 at 09:12:11AM -0500, Matthew Wilcox wrote:
> > On Wed, Dec 10, 2014 at 06:03:47AM -0800, Christoph Hellwig wrote:
> > > What is the status of this patch set?
> >
> > I have no outstanding bug reports against it. Linus told me that he
> > wants to see it come through Andrew's tree. I have an email two weeks
> > ago from Andrew saying that it's on his list. I would love to see it
> > merged since it's almost a year old at this point.
>
> And since then another month and aother merge window has passed. Is
> there any way to speed up merging big patch sets like this one?

I took a look at dax last time and found it to be unreviewable due to
lack of design description, objectives and code comments. Hopefully
that's been addressed - I should get back to it fairly soon as I chew
through merge window and holiday backlog.

> Another one is non-blocking read one that has real life use on one
> of the biggest server side webapp frameworks but doesn't seem to make
> progress, which is a bit frustrating.

I took a look at pread2() as well and I have two main issues:

- The patchset includes a pwrite2() syscall which has nothing to do
with nonblocking reads and which was poorly described and had little
justification for inclusion.

- We've talked for years about implementing this via fincore+pread
and at least two fincore implementations are floating about. Now
along comes pread2() which does it all in one hit.

Which approach is best? I expect fincore+pread is simpler, more
flexible and more maintainable. But pread2() will have lower CPU
consumption and lower average-case latency.

But how *much* better is pread2()? I expect the difference will be
minor because these operations are associated with a great big
cache-stomping memcpy. If the pread2() advantage is "insignificant
for real world workloads" then perhaps it isn't the best way to go.

I just don't know, and diligence requires that we answer the
question. But all I've seen in response to these questions is
handwaving. It would be a shame to make a mistake because nobody
found the time to perform the investigation.

Also, integration of pread2() into xfstests is (or was) happening and
the results of that aren't yet known.

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