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SubjectStrange performance behavior of 2.4.0-test9
Hi,
I found very odd performance behavior of 2.4.0-test9 on a large SMP
server, and I want some clues to investigate it.

The workload is WebBench,
Machine is:
8way PII-Xeon 450MHz 2MB cache with Profusion chip-set,
1GB Main memory with no highmem,
using 4 eepro100 networks.

As shown in the table below, test8 is very preferable in both absolute
performance and scalability.

WebBench performance results.
Req/sec(4cpu->8cpu)
--------------------------
2.4.0-test1 2816->3702 (31%up)
2.4.0-test8 4006->5287 (63%up)
2.4.0-test9 3669->2193 (40%down)
(4cpu means two CPUs for each bus.)
But the performance of test9 shows following problems:

1) At the 8 cpu configuration, test9 shows extremely inferior
performance.
2) on test8, 8-cpu configuration shows about 2/3 performance of 4-cpu.
Other experiment shows, as # of cpu increases from 4 to 8, the
performance gradually decreases.
CPUs Req/s
4cpu 3660
5cpu 3070
6cpu 2730
7cpu 2611
8cpu 2210

The reason of the performance degradation is increase of idle-time,
and increase of context switches (2.2x @4cpu, 3.1x @8cpu).


The following table shows one minute average of "vmstat 1".

configuration Req/s | r b intr c-sw user os idle
------------------------+--------------------------------------------------
2.4.0t1 4cpu 2816 | 60 0 28877 5103 18 82 0
8cpu 3702 | 37 0 33495 16387 14 86 0
2.4.0t8 4cpu 4066 | 57 0 38743 8674 27 73 0
8cpu 5287 | 33 0 40755 30626 21 78 0
2.4.0t9 4cpu 3669 | 53 3 35822 18898 25 73 2
8cpu 2193 | 5 8 22114 94609 9 52 39
In the experiments, enough free memory was keeped,
no swap-in/out happened, almost no block-in/out occured.
interrups is roughly propotional to the performance.

Performance down in 2.4.0-t9 4cpu is not so prominent but actually some
processes are blocked running by some reason.
And as the number of CPUs increases the performance degrades.
On 8cpu, only 1/10 of the processes are runnable.

I've tried test10-pre5 but nothing improved.

Does anybody tell me what I should do?
--
Computer Systems Laboratory, Fujitsu Labs.
kumon@flab.fujitsu.co.jp
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