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

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Date:	Thu, 8 Jan 2015 11:28:40 -0500
From:	Milosz Tanski <milosz@...in.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v12 00/20] DAX: Page cache bypass for filesystems on
 memory storage

On Tue, Jan 6, 2015 at 3:47 AM, Andrew Morton <akpm@...ux-foundation.org> wrote:
> On Mon, 5 Jan 2015 10:41:43 -0800 Christoph Hellwig <hch@...radead.org> 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.
>

Andrew I  got busier with my other job related things between the
Thanksgiving & Christmas then anticipated. However, I have updated and
taken apart the patchset into two pieces (preadv2 and pwritev2). That
should make evaluating the two separately easier. With the help of
Volker I hacked up preadv2 support into samba and I hopefully have
some numbers from it soon. Finally, I'm putting together a test case
for the typical webapp middle-tier service (epoll + threadpool for
diskio).

Haven't stopped, just progressing on that slower due to external factors.

P.S: Sorry for re-send. On the road and was using gmail to respond
with... it randomly forgets plain-text only settings.

-- 
Milosz Tanski
CTO
16 East 34th Street, 15th floor
New York, NY 10016

p: 646-253-9055
e: milosz@...in.com
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