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From Brian Pane <brian.p...@cnet.com>
Subject Re: mod_mem_cache bad for large/busy files (Was: [PATCH] remove some mutex locks in the worker MPM)
Date Fri, 03 Jan 2003 05:43:17 GMT
On Thu, 2003-01-02 at 21:21, David Burry wrote:
> ----- Original Message -----
> From: "Brian Pane" <brian.pane@cnet.com>
> Sent: Thursday, January 02, 2003 2:19 PM
> >
> > For large files, I'd anticipate that mod_cache wouldn't provide much
> benefit
> > at all.  If you characterize the cost of delivering a file as
> >
> >    time_to_stat_and_open_and_close +
> time_to_transfer_from_memory_to_network
> >
> > mod_mem_cache can help reduce the first term but not the second.  For
> small
> > files, the first term is significant, so it makes sense to try to optimize
> > away the stat/open/close with an in-httpd cache.  But for large files,
> where
> > the second term is much larger than the first, mod_mem_cache doesn't
> > necessarily
> > have an advantage.
> 
> Unless... of course, you're requesting the same file dozens of times per
> second (i.e. high hundreds of concurrent downloads per machine, because it
> takes a few minutes for most people to get the file).... then caching it in
> memory can help, because your disk drive would sit there thrashing
> otherwise.  If you don't have gig ethernet don't even worry you won't see
> the problem really, ethernet will be your bottleneck.  What we're trying to
> do is get close to maxing out a gig ethernet with these large files without
> the machine dying...

Definitely, caching the file in memory will help in this scenario.
But that's happening already; the filesystem cache is sitting
between the httpd and the disk, so you're getting the benefits
of block caching for oft-used files by default.


> > What sort of results do you get if you bypass mod_cache and just rely on
> > the Unix filesystem cache to keep large files in memory?
> 
> Not sure how to configure that so that it will use a few hundred megs to
> cache often-accessed large files... but I could ask around here to more
> solaris-knowledgable people...

In my experience with Solaris, the OS is pretty proactive about
using all available memory for the filesystem cache by default.
One low-tech way you could check is:
  - Reboot
  - Run something to monitor free memory (top works fine)
  - Run something to read a bunch of your large files
    (e.g., "cksum [file]").
In the third step, you should see the free memory decrease by
roughly the total size of the files you've read.

Brian



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