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Date: Tue, 19 Feb 2013 12:30:02 +1100
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To: Brendon Colby <brendon@newgrounds.com>
Cc: internals@lists.php.net
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Subject: Re: [PHP-DEV] PHP causing high number of NFS getattr operations?
From: jwrong@gmail.com (Kevin Yung)

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Hi Brendon,

In our environment, we use NFS for shared storage, we are using APC as well
with stat=3D0. In our setting, we also experiencing high number of stat()
calls on our file system. My initial finding of this problem is we enabled
the open_basedir setting. And there is already a bug report for this,
https://bugs.php.net/bug.php?id=3D52312

We tested the issue in 5.2.x, 5.3.x and 5.4.x, all of them experiencing
same issue.

I m not sure if this is a same problem as yours. But I do hope the
community can find out a good solutions to handle the balance of better
security and good performance.

Cheers

On Tue, Feb 19, 2013 at 7:26 AM, Brendon Colby <brendon@newgrounds.com>wrot=
e:

> Greetings,
>
> This seemed like the appropriate list to discuss this - let me know if it
> isn't.
>
> We have several Apache 2.2 / PHP 5.4 / APC 3.1.13 servers all serving
> mostly PHP over NFS (we have separate servers for static content). I
> have been trying to figure out why we're seeing so many getattr
> requests from our Apache/PHP servers. I think I've narrowed it down to
> how PHP/APC gets information about a file to see if it has been
> changed since it was last cached:
>
> https://bugs.php.net/bug.php?id=3D59372
>
> Rasmus said:
>
> "Just because you see an open() syscall doesn't mean the cache isn't
> used. The open() is there because PHP by default does open+fstat for
> performance reasons. If you look carefully you will see there is just
> an open() and no read() calls. So it opens the file, uses the stat
> struct from the open fd to get the inode and fetches the op_array from
> the opcode cache. This can be made more efficient, but this report is
> incorrect in assuming the cache isn't being used."
>
> This is exactly what I'm seeing when I strace an httpd process -
> mostly open() and fstat() calls w/o any read() calls. This makes sense
> to me, however, the problem with this is as far as I understand it,
> that an open() on an NFS file system causes an automatic gettattr()
> call to the server. Doing this completely bypasses the NFS attribute
> cache, which is why we're seeing so many getattr requests to our NFS
> server despite having an attribute cache timeout of 60 seconds. For
> those familiar with NFS, we could just as well disable our attribute
> cache at this point, which is almost never recommended because of
> performance reasons.
>
> I found a good explaination of why this happens in this Redhat document:
>
>
> http://www.redhat.com/rhecm/rest-rhecm/jcr/repository/collaboration/jcr:s=
ystem/jcr:versionStorage/5e75391d7f00000110a97b91452f0cee/1/jcr:frozenNode/=
rh:pdfFile.pdf
>
> "Avoid unnecessarily opening and closing of files. If you access a
> file frequently, opening and closing that file triggers the
> open=C2=ADto=C2=ADclose cache
> consistency mechanism in the NFS file system.  Triggering this
> mechanism causes NFS getattr requests to be generated, which can slow
> performance. By keeping the file open, traffic is kept to a minimum."
>
> I've tried doing apc.stat=3D0 and about every other combination of
> options, but PHP still does an open()+3 fstat() commands on every
> single page load no matter what I do. We have autoload set, so that
> equates to several automatic open() (and subsequent NFS getattr())
> calls with every single PHP request. To give you an idea what kind of
> load this is generating for us:
>
> -Out of 7,000 total NFS operations per second right now:
> -About 45% are getattr calls
> -Our Apache/PHP servers are generating 52% of all NFS calls
> -An average of 82% of those calls are getattr requests
> -As a comparison, our far busier content servers (many times the req/s
> of the Apache/PHP servers) are generating half the number of NFS
> requests our PHP servers are, and 98% or so of those requests are read
> requests with just 1% getattr
>
> I am pretty proficient with patching/compiling/profiling, but sadly I
> am no C coder. I've tried digging into this myself but I'm pretty out
> of my element here. I'm curious if anyone on this list could give me
> any pointers on what I can do to change this behavior and reduce the
> load on our NFS server. If we can't change PHP behavior, we may have
> to abandon NFS for our PHP files due to the load PHP appears to be
> generating on our NFS server. I'd hate to do this, so please let me
> know if any of you have any ideas as to what I can do to change this
> behavior.
>
> Brendon
>
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