Date: Sun, 14 Feb 1999 20:10:27 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Jaye Mathisen <mrcpu@internetcds.com> Cc: hackers@FreeBSD.ORG Subject: Re: Processor affinity? Message-ID: <199902150410.UAA12131@apollo.backplane.com> References: <Pine.NEB.3.95.990214183141.27256P-100000@schizo.cdsnet.net>
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:Noticed somethign a little odd with a 3.1-stable supped a few days ago.
:
:There were only 3 processes of any significance running on a dual
:processor P6.
:
:mysql, top, and a perl script pumping kazillions of small records into the
:mysql database.
:
:The perl script was basically burning 99.9% of the CPU. Mysql was chewing
:up 40-60% of the CPU.
:
:For some reason that on the suface does not make sense to me, the perl
:script moved back and forth among the CPU's, just about like clockwork,
:every update or so of top.
:
:This seems odd to me, in that given the size of the program (small), and
:the amount of CPU burning, it would be better if the perl process stayed
:stuck to 1 CPU to maximise the use of the cache (512k) of that CPU,
:instead of having to wander around.
:
:Is there some way to set processor affinity on a process? Or some tweak
:to some sysctl variable somewhere that I can take advantage of?
Doing processor affinity properly is very difficult. It isn't as simple
as trying to keep a process on a specific cpu. Whatever algorithm is
chosen must deal with the situation under a greater process load. Often,
as on IRIX 6.1 boxes, affinity could make things worse rather then better
by unbalancing the cpu's. Processor affinity makes sense when you have
a lot of processors ( you can schedule a process to a group of cpu's to
maintain reasonable balancing across the system), but doesn't make much
sense if you only have 2-4.
Note that processor affinity scheduling is different from hard-assigning
a process to a processor. Even so, there are very few circumstances where
even hard-assigning will do a better job then letting the scheduler do it.
-Matt
Matthew Dillon
<dillon@backplane.com>
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