linux-kernel - SMP Affinity and nice

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Date:	Wed, 23 Aug 2006 11:24:33 -0400 (EDT)
From:	"Rich Paredes" <rparedes@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: SMP Affinity and nice

I am trying to come to an understanding as to why 1 process is getting
less cpu time than identical processes with a higher "nice" value.
Server has 2 physical processors with hyperthreading (cpu 0,1,2,3)

I am starting 5 processes that perform a square root loop to max out a
cpu.  They use the exact same code but are renamed for identification:
cpumax1, cpumax2, cpumax3, cpumax4, cpumax5
I start in order:
1.  nice -n 10 cpumax1
2.  nice -n 10 cpumax2
3.  nice -n 10 cpumax3
4.  nice -n 10 cpumax4
5.  nice -n 0 cpumax5

Here is the top output:
 PR  NI S %CPU    TIME+  P COMMAND
 35  10 R 99.9   1:46.90 3 cpumax1
 35  10 R 99.9   1:41.01 1 cpumax3
 35  10 R 99.9   1:39.48 0 cpumax4
 25   0 R 66.9   1:03.13 2 cpumax5
 35  10 R 33.0   0:39.30 2 cpumax2

cpumax1 is using processor 3, 99%
cpumax2 is using processor 2, 33%
cpumax3 is using processor 1, 99%
cpumax4 is using processor 0, 99%
cpumax5 is using processor 2, 66%

So since cpumax5 has a lower nice value and thus a higher priority (25 in
this case), shouldn't it be given it's own cpu.   If I give cpumax5 a nice
value of -20, it does start using it's own cpu.  I don't want to manage
cpu affinity via taskset command.

My explanation would be that since the scheduler tries to limit cpu
affinity, the nice value of 0 isn't enough to get the scheduler to move
this process to another processors run queue.  I could be totally wrong
here though.

I should also note here that this test is totally dependent on the order
of startup.  If I start cpumax5 first with a nice value of 0 before the
other 4, it will get it's own cpu:
 PR  NI S %CPU    TIME+  P COMMAND
 35  10 R 99.9   1:00.03 3 cpumax2
 25   0 R 99.9   1:08.01 1 cpumax5
 35  10 R 99.9   1:03.69 2 cpumax1
 35  10 R 50.3   0:29.02 0 cpumax3
 35  10 R 49.6   0:26.37 0 cpumax4

I just want to understand this better.  Thanks.
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