On 12/14/2010 01:54 PM, David S. Ahern wrote:
On 12/14/10 12:49, Anthony Liguori wrote:
But that doesn't tell you what the impact is in real world workloads.
Before we start pushing all device emulation into the kernel, we need to
quantify how often gettimeofday() is really called in real workloads.
The workload that inspired that example program at its current max load
calls gtod upwards of 1000 times per second. The overhead of
gettimeofday was the biggest factor when comparing performance to bare
metal and esx. That's why I wrote the test program --- boils a complex
product/program to a single system call.
So the absolute performance impact was on the order of what?
The difference in CPU time of a light weight vs. heavy weight exit
should be something like 2-3us. That would mean 2-3ms of CPU time at a
rate of 1000 per second.
That should be pretty much in the noise.
There are possibly second order effects that might make a large impact
such as contention with the qemu_mutex. It's worth doing
experimentation to see if a non-mutex acquiring fast path in userspace
also resulted in a significant performance boost.
Regards,
Anthony Liguori
David
Regards,
Anthony Liguori
What's the relative speed of the in-kernel pmtimer compared to the PIT?
David
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