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From Vivek Sadananda Pai <vi...@cs.rice.edu>
Subject Re: fix for hybrid server problems.
Date Mon, 17 May 1999 13:15:35 GMT
On Sun, 9 May 1999, Dean Gaudet <dgaudet@arctic.org> wrote:
> 
> So far my largest concern, which is the same as my concern with entirely
> select-based servers, is disk i/o.  This all works wonderfully if you
> rarely have to page from disk.  But for servers with large working sets,
> aggregating like this hurts because you have only a single i/o request
> outstanding at a time... with multiple processes we alleviate some of this
> problem... there are probably other options -- but I think this is
> something we can deal with when we get there. 

I found two problems with purely select-based servers with regard to
having only a disk single I/O available. The first, of course, is that
you can't parallelize disk accesses (scheduling or multiple disks),
but more importantly, while the process is blocked, no other
user-level processing occurs. 

It turns out to be relatively simple to adapt a select-based server to
use other processes for all disk data and metadata accesses, and
that's one of the goals of the Flash server - measurements suggest
that its performance across cache-friendly or even disk-bound
workloads is competitive with any other model. 

If Apache goes partially select-based, the Flash model might be useful
to address the disk problems. Details are in the upcoming Usenix paper,
available at http://www.cs.rice.edu/~vivek/flash99/

-Vivek


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