On May 28, 2009, at 12:55 PM, CrystalCracker wrote:
Thanks a lot for the response. I will do some ab and siege tests to verify it, but I am trying to conceptually understand it first. <IfModule prefork.c> StartServers 5 MinSpareServers 5 MaxSpareServers 20 MaxClients 250 MaxRequestsPerChild 4000
This seems way low: you would be killing off children after they handled 4,000 connections (connections, not requests making this a slight misnomer). Unless you have something that is leaking memory like a sieve, you can raise this significantly or omit it alltogether.
MaxClients is also a misnomer: see below.
</IfModule>I have at least 20 active apache threads (ps -ef | grep httpd), average is about 40 threads and goes upto 70 at the peak. Does the above setting soundsresonable?
Besides the above: sure. Apache is quite apt at managing its pool of children. According to some performance lore, memory allocation might happen more efficiently if you start your maximum number of children at startup and leave them running. So,
StartServers 250 MinSpareServers 0 MaxSpareServers 250 MaxClients 250and no MaxRequestsPerChild (if you have no memory leaks). Doug's note on memory usage is spot-on: make sure you never run out of physical RAM by limiting that MaxClients value. Note that mod_perls or mod_php can make child processes right big.
Doug Bell-4 wrote:On May 27, 2009, at 11:14 PM, CrystalCracker wrote:Can anyone explain me how exactly prefork works?
Not really, but I can have a stab.The parent process, which typically runs as root, binds to a socket (usually port 80 or 443). It spawns children, which inherit the open file descriptor for the socket, and change uid and gid to the unprivileged user and group. The children construct a pollset of the listener file descriptors (if there is more than one listener) and watch for activity on it/them. If activity is found, the child calls accept() on the active socket and handles the connection. When it is done with that, it returns to watching the pollset (or listener file descriptor).
Since multiple children are active and they all inherited the same socket file descriptor(s), they will be watching the same pollset. An accept mutex allows only a single child to actually watch the pollset, and once that has found an active socket it will unlock the mutex so the next child can start watching the pollset. If there is only a single listener, that accept mutex is not used and all children will hang in accept().
Children record in a shared memory area (the scoreboard) when they last served a request. Idle children may be killed by the parent process to satisfy MaxSpareServers. If too few children are idle, the parent will spawn children to satisfy MinSpareServers.
On a quiet server, arrival of a new connection will be relatively scarce, and most children will be camping on the accept mutex, or in accept(). On a busy server, the poll or naked accept() will return immediately and few if any children are idle. On an extremely busy server, the parent will keep spawning children until it hits MaxClients. Arriving connections will back up in the kernel before they are accepted by child processes (see the ServerBacklog directive). This is why MaxClients is a misnomer: it *only* applies to the number of child processes spawned and does not take into account either the backlog, or the fact that most HTTP traffic is short-lived and bursty, and clients will make many short-lived requests that can be handled by any of the child processes.
There is of course more to it, but this is pretty much what the Prefork MPM does.
S.
StartServers 2 MaxClients 150 MinSpareThreads 25 MaxSpareThreads 75 ThreadsPerChild 25 ServerLimit 16Prefork works by spawning one process to handle one connection. Sohere, Apache can spawn a maximum of 150 httpd processes (MaxClients ismaximum number of concurrent connections, which in prefork works out to number of processes).I think I got confused between the terms - server, process and thread. If prefork is a single thread process, what does that ThreadsPerChild mean for prefork?Prefork is not threaded, so the thread config values do not apply.Lets say, I wan't to support 250 active sessions logged into my site, what would be a tentative settings to meet the load?"Active sessions" is difficult to measure. Let's say a person requestsa page which takes 10 seconds, then looks at it for 20 seconds (on average). This means that you need one httpd process for three users(the other two people using the 20 seconds the first user isn't using).So to get 250 users from these assumptions, we need MaxClients 74. I would use "siege" to measure your performance. It has settings for how many concurrent users, and can attempt to simulate randomness and "user is reading the page" time. Doug Bell -- Senior Developer, Plain Black Corp. [ http://plainblack.com ] ---------------------------------------------------------------------The official User-To-User support forum of the Apache HTTP Server Project.See <URL:http://httpd.apache.org/userslist.html> for more info. To unsubscribe, e-mail: users-unsubscribe@xxxxxxxxxxxxxxxx " from the digest: users-digest-unsubscribe@xxxxxxxxxxxxxxxx For additional commands, e-mail: users-help@xxxxxxxxxxxxxxxx-- View this message in context: http://www.nabble.com/How-does-Prefork-work--tp23754946p23768888.htmlSent from the Apache HTTP Server - Users mailing list archive at Nabble.com.---------------------------------------------------------------------The official User-To-User support forum of the Apache HTTP Server Project.See <URL:http://httpd.apache.org/userslist.html> for more info. To unsubscribe, e-mail: users-unsubscribe@xxxxxxxxxxxxxxxx " from the digest: users-digest-unsubscribe@xxxxxxxxxxxxxxxx For additional commands, e-mail: users-help@xxxxxxxxxxxxxxxx
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