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=59372 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:system/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 opentoclose 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=0 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 -- PHP Internals - PHP Runtime Development Mailing List To unsubscribe, visit: http://www.php.net/unsub.php
