Re: process priorities and performance

[James G Smith <JG...@TAMU.Edu>]

"Jim Helm" <jj...@att.net> wrote:
>Everything I've read as an SA (for Solaris at least - though I would
>expect the other *nices to be similar) was to never set a user space
>(non O/S) process to less than -15.  Other than that, it's another of
>those YMMV, measure before and after, and if it helps great.  Trying to
>second guess process schedulers is a tricky business though, and you
>really need to intimately know how your system behaves before trying it.
>
>> -----Original Message-----
>>
>> Alexey Zvyagin has suggested a use of Unix process priorities 
>> to improve 
>> the performance of the web services during the peak hours:
>> 
>> Alex writes:
>> ---------------------------------------------------------

 [snip]

>> The CPU priorities help to handle an increased traffic on the 
>> overloaded 
>> server.
>> ---------------------------------------------------------

I think the key here is the fact that the system is
overloaded/overcommitted.  Too many processes are competeing for
CPU.

Putting my SA hat on, I would say the processes need to be split
across multiple pieces of hardware or a new machine needs to replace
the current system.  That's the only real long-term solution for a
system in this state.

The priorities as described help set a relative importance between
the processes--the front-end is more important than the database with
the larger server processes in the middle.  This is not
unreasonable.  But a serious solution to the problem of an overloaded
system is to put in more system.  The priorities might be helpful for
the few minutes/hours/days needed to get the new hardware on the
floor.

Imho, a web server should be designed for the expected peak normal
usage plus a fudge factor thrown in for safety and multiplied by a
small integer greater than one for growth (I'm optimistic).

Conclusion of my thoughts: putting in a blurb about priorities being
able to set relative importance of processes is fine, but don't cast
it as a solution (just as swap space is not a solution to constrained
memory).  It's a bit of a band-aid that can help until the problem
can be fixed.
-- 
James Smith <JG...@TAMU.Edu>, 979-862-3725
Texas A&M CIS Operating Systems Group, Unix