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From dean gaudet <dgaudet-list-new-ht...@arctic.org>
Subject Re: [PATCH] Problems with MPM threaded
Date Tue, 17 Jul 2001 08:13:17 GMT
On Sat, 14 Jul 2001 rbb@covalent.net wrote:

> Having multiple processes each with multiple threads provides for FAR
> more robustness than just a single process with multiple threads.

ya know, i'm not really convinced of the desirability of this explanation
anymore.  maybe the hypothetical buggy 3rd party modules should be fixed
rather than hacked around.  and they won't be fixed as long as apache
continues to hide the problem.

however i can provide two more justifications which are pretty weak:

- userland thread libraries need multiple processes in order to get disk
  i/o parallelism (and multi-cpu parallelism)

- suppose you configure apache with exactly the number of children as
  you have processors.  and suppose that somehow magically the CPU
  requirements of each process' threads is about even.  then a good
  (kernel) scheduler will likely migrate each process to its own CPU and
  time slice just the threads within that process.  this will tend to
  decrease the overall memory system traffic (because L2 caches won't be
  competing for the same process' memory), and increase the TLB hit rate.
  it may even be possible to avoid interprocessor interrupts every
  time the memory map is changed (to make sure that remote TLBs are
  up to date).

  that's a lot of conditionals to get right.

-dean


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