Server optimizations for php

[Arvind]

On Jan 24, Atif Ghaffar wrote:
>But this is now. I want to scale the system to be tranquille for the next 2
>years.
>So would a 2 - 4  of these servers (
>http://www.sun.com/servers/coolthreads/t5140/ ) with 32GB RAM be overkill
>
>Would these more more suitable
>2-4 http://www.sun.com/servers/x64/x4150/ with 8 cores and 32-64GB RAM.
>
>From price level I think they will come to more or less the same.
>I like the idea to take some extra RAM and put the tmp files, cached opcode
>files  and smarty generated template in the RAM.
>With the low prices of RAM it wont make  a hole in the budget neither.
>
>Oh and for page views we have around 0.5M at the moment but this is growing
>quiet fast.


Atif, I'd suggest you do not go about buying beefier machines without
strong justifications. 

RAM scaling
=========
I've worked with many websites that do millions of requests a day in
apache+php and am yet to see any application that needs even 32GB of
RAM. When you are trying to consolidate web servers, the following math
*does not* hold true in general:
'x' machines with 'y' amount of RAM needs to be replaced by a single
machine with x*y amount of memory.

In a well designed web application, large portions of memory is used by
shared resources (code segments, shared cache etc. etc.) The amount of
memory required for actual connection handling is very lowThis is all
the more true in the case of nginx as compared to apache. (not even
close to 1MB per request). So, memory needs to be scaled less than
linearly when you consolidate servers. The only exception is when both
of the following conditions hold true:
1) Large amounts of memory is used purely for application caching
2) There is some sort of a well defined partitioning of cached data
across nodes to ensure that the overlap is minimal

Where nginx should make a huge difference over apache: If your php
application is I/O bound and a large number of connections need to be
held in waiting state, then nginx should help in better memory and cpu
usage.

CPU scaling
===========
As you might know, adding more CPUs scales less than lineraly. Much of
the problems arise from lock contentions. So unless you know that your
entire application stack is known to scale well on a large number of
cores, do not try to use something like a 64 core machine; the results
might be devastating