Ooops, what I meant to say was that 2 threads bound to one (hyperthreaded) cpu does *NOT* cause the storm, even on an smp xeon.

Therefore, the context switches may be a result of cache coherency related delays. (2 threads on one hyperthreaded cpu presumably have tightly coupled 1,l2 cache.)

On Apr 20, 2004, at 1:02 PM, Paul Tuckfield wrote:

I tried to test how this is related to cache coherency, by forcing affinity of the two test_run.sql processes to the two cores (pipelines? threads) of a single hyperthreaded xeon processor in an smp xeon box.

When the processes are allowed to run on distinct chips in the smp box, the CS storm happens. When they are "bound" to the two cores of a single hyperthreaded Xeon in the smp box, the CS storm *does* happen.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ er, meant *NOT HAPPEN*

I used the taskset command: taskset 01 -p <pid for backend of test_run.sql 1> taskset 01 -p <pid for backend of test_run.sql 1>

I guess that 0 and 1 are the two cores (pipelines? hyper-threads?) on the first Xeon processor in the box.

I did this on RedHat Fedora core1 on an intel motherboard (I'll get the part no if it matters)

during storms : 300k CS/sec, 75% idle (on a dual xeon (four core)) machine (suggesting serializing/sleeping processes)
no storm: 50k CS/sec, 50% idle (suggesting 2 cpu bound processes)

Maybe there's a "hot block" that is bouncing back and forth between caches? or maybe the page holding semaphores?

On Apr 19, 2004, at 5:53 PM, Tom Lane wrote:

I wrote:
Here is a test case.

Hmmm ... I've been able to reproduce the CS storm on a dual Athlon, which seems to pretty much let the Xeon per se off the hook. Anybody got a multiple Opteron to try? Totally non-Intel CPUs?

It would be interesting to see results with non-Linux kernels, too.

regards, tom lane

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