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Gmane
From: Andy Lutomirski <luto <at> amacapital.net>
Subject: Re: page fault scalability (ext3, ext4, xfs)
Newsgroups: gmane.comp.file-systems.ext4
Date: Thursday 15th August 2013 22:26:09 UTC (over 4 years ago)
On Thu, Aug 15, 2013 at 3:18 PM, Dave Chinner  wrote:
> On Thu, Aug 15, 2013 at 02:43:09PM -0700, Andy Lutomirski wrote:
>> On Thu, Aug 15, 2013 at 2:37 PM, Dave Chinner
>>  wrote:
>> > On Thu, Aug 15, 2013 at 08:17:18AM -0700, Andy Lutomirski wrote:
>> >> My behavior also means that, if an NFS
>> >> client reads and caches the file between the two writes, then it will
>> >> eventually find out that the data is stale.
>> >
>> > "eventually" is very different behaviour to the current behaviour.
>> >
>> > My understanding is that NFS v4 delegations require the underlying
>> > filesystem to bump the version count on *any* modification made to
>> > the file so that delegations can be recalled appropriately. So not
>> > informing the filesystem that the file data has been changed is
>> > going to cause problems.
>>
>> We don't do that right now (and we can't without utterly destroying
>> performance) because we don't trap on every modification.  See
>> below...
>
> We don't trap every mmap modification. We trap every modification
> that the filesystem is informed about. That includes a c/mtime
> update on every write page fault. It's as fine grained as we can get
> without introducing serious performance killing overhead.
>
> And nobody has made any compelling argument that what we do now is
> problematic - all we've got is a microbenchmark doesn't quite scale
> linearly because filesystem updates through a global filesystem
> structure (the journal) don't scale linearly.

I don't personally care about scaling.  I care about sleeping in write
faults, and starting journal transactions sleeps, and this is an
absolute show-stopper for me.  (It's a real-time latency problem, not
a throughput or scalability thing.)

>
>> >> The current behavior, on
>> >> the other hand, means that a single pass of mmapped writes through
the
>> >> file will update the times much faster.
>> >>
>> >> I could arrange for the first page fault to *also* update times when
>> >> the FS is exported or if a particular mount option is set.  (The ext4
>> >> change to request the new behavior is all of four lines, and it's
easy
>> >> to adjust.)
>> >
>> > What does "first page fault" mean?
>>
>> The first write to the page triggers a page fault and marks the page
>> writable.  The second write to the page (assuming no writeback happens
>> in the mean time) does not trigger a page fault or notify the kernel
>> in any way.
>
> IIUC, you are saying is that you'll maintain the current behaviour
> (i.e. clean->dirty does a timestamp update) if the filesystem
> requires it? So the default behaviour of any filesystem that
> supports NFSv4 is going to behave as it does now?
>
> If that's the case, why bother changing anything as nfsv4 is the
> default version that the kernel uses? (I'm playing devil's advocate
> here).

Because the performance sucks right now.  I'd like to fix it without
breaking things, and I think I can fix it while actually improving the
semantics.

>
>> In current kernels, this chain of events won't work:
>>
>>  - Server goes down
>>  - Server comes up
>>  - Userspace on server calls mmap and writes something
>>  - Client reconnects and invalidates its cache
>>  - Userspace on server writes something else *to the same page*
>>
>> The client will never notice the second write, because it won't update
>> any inode state.
>
> That's wrong. The server wrote the dirty page before the client
> reconnected, therefore it got marked clean.

Why would it write the dirty page?  Is the client's NFSv4 request
forcing the server to scan for dirty ptes or pages?  If so, can you
point me to that code?  I can probably make it work deterministically.

> The second write to the
> server page marks it dirty again, causing page_mkwrite to be
> called, thereby updating the timestamp/i_version field. So, the NFS
> client will notice the second change on the server, and it will
> notice it immediately after the second access has occurred, not some
> time later when:
>
>> With my patches, the client will as soon as the
>> server starts writeback.
>
> Your patches introduce a 30+ second window where a file can be dirty
> on the server but the NFS server doesn't know about it and can't
> tell the clients about it because i_version doesn't get bumped until
> writeback.....

I claim that there's an infinite window right now, and that 30 seconds
is therefore an improvement.

--Andy
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