Re: How does Prefork work?

[Doug Bell <doug@xxxxxxxxxxxxxx>]

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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