Newsgroups: php.internals
Path: news.php.net
Xref: news.php.net php.internals:38985
Mailing-List: contact internals-help@lists.php.net; run by ezmlm
Received-SPF: error (pb1.pair.com: domain lerdorf.com from 204.11.219.139 cause and error)
Message-ID: <487CF99F.8090701@lerdorf.com>
Date: Tue, 15 Jul 2008 12:25:19 -0700
User-Agent: Thunderbird 2.0.0.4 (Macintosh/20070604)
MIME-Version: 1.0
To: Amir Hardon <amirh@metacafe.com>
CC: PHP Internals List <internals@lists.php.net>
References: <1216133436.6875.35.camel@amirh>  <487CEF26.7030802@lerdorf.com> <1216149220.16451.10.camel@amirh>
In-Reply-To: <1216149220.16451.10.camel@amirh>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Subject: Re: [PHP-DEV] lstat call on each directory level
From: rasmus@lerdorf.com (Rasmus Lerdorf)

Amir Hardon wrote:
> On Tue, 2008-07-15 at 11:40 -0700, Rasmus Lerdorf wrote:
>> Amir Hardon wrote:
>> > I've noticed a weird behavior when doing file access from PHP:
>> > PHP seems to make an lstat call on each of the parent directories of the
>> > accessed file, for example see this script:
>> > 
>> > <?php
>> > $fp=fopen("/var/www/metacafe/test","r");
>> > fclose($fp);
>> > ?>
>> > 
>> > When running with strace -e lstat I see this:
>> > lstat("/var", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
>> > lstat("/var/www", {st_mode=S_IFDIR|0755, st_size=12288, ...}) = 0
>> > lstat("/var/www/metacafe", {st_mode=S_IFDIR|0755, st_size=4096, ...}) =
>> > 0
>> > lstat("/var/www/metacafe/test", 0x7fbfff9b10) = -1 ENOENT (No such file
>> > or directory)
>> > 
>> > Measuring total syscalls time for an apache process on a production
>> > server, I found out
>> > that ~33% of the time it spends in syscalls is spent on lstat.
>> > 
>> > I did a pretty deep web search on the issue and came out with nothing.
>> > I'll also note that I did a small experiment - moving our root portal
>> > folder to /,
>> > this gave an amazing performance improvement!
>> > 
>> > So my questions are:
>> > What is the reason for doing these lstat calls?
>> > How can it be disabled? if not by configuration, maybe by patching php
>> > (can you direct me to where is this being done in php's source?)
>>
>> That's a realpath() call and it should be getting cached by the realpath 
>> cache, so if you are seeing these on every request, try increasing your 
>> realpath_cache size in your .ini.  Without checking the realpath, you 
>> would be able to circumvent open_basedir checking really easily with a 
>> symlink.
>>
>> -Rasmus
> 
> I've already increased the realpath_cache to the point it didn't give 
> any more benefit(And it did give benefit),
> but there are still many lstat calls, and still placing our portal dir 
> in the root directory gave a huge performance benefit(After fine-tuning 
> realpath_cache).
> We don't use open_basedir.
> 
> I think it might be wise to make this dir check configurable, as the 
> performance impact is major.
> Anyway - can you please direct me to the place where this check is made 
> in php's source, so I'll be able to disable it manually?

Well, it is used in other places too, like in figuring out _once paths. 
  Including the same file using different paths still needs to be caught.

Are you calling clearstatcache() manually anywhere?  That blows away the 
entire realpath cache and completely destroys your performance, so you 
will want to avoid doing that very often.

-Rasmus