Dual-core CPU vs. very large cache

[Joseph Koshy]

> Our current Dells have 2M cache, and I'm trying to determine
> whether the 8M cache will make a significant difference or
> not.  Can someone recommend a testing procedure for
> determining whether adding cache is worthwhile or not?
> I can simulate a test load at any time, but I don't know how
> to tell whether the cache is the bottleneck of the CPU or not.

You didn't say which version of FreeBSD you are running.

If this is 6.X, you could run pmcstat(8) in a system-wide
counting mode and measure cache behaviour directly.

Briefly,

  # (compile, boot a kernel with options HWPMC_HOOKS)
  # kldload hwpmc
  # pmcstat -w 1 -s dc-misses test-load

See pmc(3) and the CPU vendor documentation referenced there
for the events your CPU can measure.  If your application
is CPU bound you may want to check for memory bus behaviour,
cache behaviour, cycles the processor was stalled and
for poor branch prediction.  If you notice anything anomalous,
you can use the hwpmc's sampling modes to find out what code
is causing the anomaly.

--
FreeBSD Volunteer,     http://people.freebsd.org/~jkoshy