What is loading my server so much?
nightrecon at hotmail.com
Fri Dec 10 15:22:43 UTC 2010
Matthew Seaman wrote:
> On 09/12/2010 11:31, Laszlo Nagy wrote:
>> Today something happened. Number of http processes went up to 200. As a
>> result, number of connections to database also went up to 200, and the
>> web server is now refusing clients with "Cannot connect to database"
>> messages (coming from PHP).
> This is a classic scenario. Some burst of traffic causes your apache to
> spawn more child processes than will all fit in RAM at one time.
> Consequently, the system starts to swap. Swapping kills performance.
> This slows everything down so much that there are always requests
> waiting for apache to process, so apache will never find any idle
> children to kill off. Result misery.
> The answer is to limit the number of child processes apache will spawn.
> Decide how much of your available RAM you can devote to Apache. Look at
> top(1) to find the maximum size apache processes grow to. The ratio of
> those two sizes is the maximum number of apache processes your system
> can support.
> Limiting the total number of apache processes sounds counter-intuitive.
> What happens when you get sufficient traffic that apache maxes out? Web
> queries will generally be queued up until there's an apache child free
> to handle them. Generally that will take from a few 10s of milliseconds
> on up -- although if you're regularly getting into a state where your
> webserver takes seconds to answer, then it's time to get more beefy
The quintessential "first-try" for this has historically been to set
keepalives to "Off". Not a solution, just an interim stop gap. If this makes
any discernible improvement (however small) it confirms the situation.
What I did was switch to the event mpm and FastCGI. Fewer processes, but
each process spawns lots of threads . Each of the threads within a process
can reuse the database connection previously established by a different
thread. Saves connection create/build-up/tear-down cycles as well as RAM.
Then throw libmemcached into the mix, if possible. To properly utilize
memcached your PHP has to have some code inserted so it will talk. This sets
aside a cached buffer pool of RAM for database connections , similar to the
connection pool you would find in use by a Java servlet container such as
Tomcat or Resin. If unable to refactor code, in lieu of this you can enable
MySQL SQL data cache. Not as effective, but if you get a lot queries which
are the same SQL it saves cycles. The amount of YMMV here would depend upon
how many queries are repetitive. If each and every query is unique it won't
So some fictitious example would be like start with some small number of
stand-by processess such as 8,16,32, etc. Each process might have a thousand
threads. When this thousand threads is reached Apache opens another process
with a thousand more. You would still want to set a max-child/max-thread
count along the lines you previously described. However, this approach gets
you more headroom at the PHP data access wall of hanging doom.
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