Re: How does Prefork work?

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On May 28, 2009, at 2:55 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
</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 sounds
resonable?


Firstly, please don't top-post. It is difficult to maintain the thread  
of conversation when you top-post.
Next:

MaxClients at 250 means that potentially 250 httpd processes can be running at once. Do you have the memory to support that many without swapping? Swapping usually ends up killing a system, especially if it's already handling a lot of disk i/o.
MaxSpareServers at 20 means that at most 20 servers will sit there  
idle. MinSpareServers means at least 5 will sit idle. These are best  
tweaked for how you want the server to respond to a mass influx of  
traffic. Does your traffic tend to be steady throughout the day or  
does it spike sharply once or more? If you have spikes, then your Min/ 
MaxSpareServers are fine as they are. If not, you might want to reduce  
the MaxSpareServers to 10 so that the memory can be reclaimed and used  
for better things.


Doug Bell-4 wrote:
On May 27, 2009, at 11:14 PM, CrystalCracker wrote:

Can anyone explain me how exactly prefork works?

StartServers 2
MaxClients 150
MinSpareThreads 25
MaxSpareThreads 75
ThreadsPerChild 25
ServerLimit 16
Prefork works by spawning one process to handle one connection. So
here, Apache can spawn a maximum of 150 httpd processes (MaxClients is
maximum 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 requests
a 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 ]




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