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SubjectRe: [Lse-tech] [RFC, PATCH] 1/5 rcu lock update: Add per-cpu batch counter
On Tue, May 25, 2004 at 07:35:20AM +0200, Manfred Spraul wrote:
> Hi,
>
> Step one for reducing cacheline trashing within rcupdate.c:
>
> The current code uses the rcu_cpu_mask bitmap both for keeping track
> of the cpus that haven't gone through a quiescent state and for
> checking if a cpu should look for quiescent states. The bitmap
> is frequently changed and the check is done by polling -
> together this causes cache line trashing.
>
.....



It looks like the patch fixes the problem that I saw.

I ran a 2.6.6+rcupatch kernel on a 512p system. Previously, this system
showed ALL cpus becoming ~50% busy when running a "ls" loop on a single
cpu. This behavior is now fixed.

The following shows the system overhead on cpus 0-9 (higher cpus are
similar):


idle system
CPU 0 1 2 3 4 5 6 7 8 9
0.70 0.21 0.19 0.17 0.21 0.20 0.20 0.19 0.18 0.19
0.72 0.21 0.18 0.17 0.21 0.21 0.21 0.20 0.18 0.19
0.70 0.20 0.18 0.17 0.22 0.20 0.19 0.20 0.19 0.20
0.71 0.19 0.18 0.17 0.21 0.20 0.19 0.20 0.18 0.19
0.71 0.23 0.19 0.17 0.21 0.20 0.19 0.20 0.19 0.20


runnin "ls" loop on cpu 4 (the number for cpu4 is the ls command - NOT overhead)
CPU 0 1 2 3 4 5 6 7 8 9
0.83 2.08 0.31 0.29 97.87 1.25 0.32 0.32 0.32 0.33
0.88 0.52 0.30 0.28 96.46 1.32 0.32 0.23 0.31 0.30
0.84 1.27 0.31 0.29 97.15 1.38 0.33 0.31 0.31 0.31
0.83 2.81 0.32 0.30 98.61 1.14 0.31 0.33 0.32 0.37
0.84 2.32 0.32 0.40 97.91 1.43 0.33 0.43 0.32 0.36

There is a small increase in system overhead on all cpus but not the 50% seen
earlier.

I dont understand, however, why the overhead increased on cpus 1 & 5. This
may be a test anomaly but I suspect something else. I'll look into that later.

I also noticed one other anomaly. /proc/interrupts shows the number of interrupts
for each cpu. Under most circumstances, /proc/interrupts shows 1024 timer
interrupts/sec on each cpu. When the "ls" script is running, the number
of timer interrupts/sec on the cpu running "ls" drops to ~650. I'm not
sure why (perhaps running too long somewhere with ints disabled).

I'll look further....



--
Thanks

Jack Steiner (steiner@sgi.com) 651-683-5302
Principal Engineer SGI - Silicon Graphics, Inc.


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