Re: [kvm-devel] performance with guests running 2.4 kernels (specifically RHEL3)

Avi Kivity
2008-05-28T10:51:18Z

David S. Ahern wrote:
> The short answer is that I am still see large system time hiccups in the
> guests due to kscand in the guest scanning its active lists. I do see
> better response for a KVM_MAX_PTE_HISTORY of 3 than with 4. (For
> completeness I also tried a history of 2, but it performed worse than 3
> which is no surprise given the meaning of it.)
>
>
> I have been able to scratch out a simplistic program that stimulates
> kscand activity similar to what is going on in my real guest (see
> attached). The program requests a memory allocation, initializes it (to
> get it backed) and then in a loop sweeps through the memory in chunks
> similar to a program using parts of its memory here and there but
> eventually accessing all of it.
>
> Start the RHEL3/CentOS 3 guest with *2GB* of RAM (or more). The key is
> using a fair amount of highmem. Start a couple of instances of the
> attached. For example, I've been using these 2:
>
> memuser 768M 120 5 300
> memuser 384M 300 10 600
>
> Together these instances take up a 1GB of RAM and once initialized
> consume very little CPU. On kvm they make kscand and kswapd go nuts
> every 5-15 minutes. For comparison, I do not see the same behavior for
> an identical setup running on esx 3.5.
>

I haven't been able to reproduce this:

> [root@localhost root]# ps -elf | grep -E 'memuser|kscand'
> 1 S root 7 1 1 75 0 - 0 schedu 10:07 ?
> 00:00:26 [kscand]
> 0 S root 1464 1 1 75 0 - 196986 schedu 10:20 pts/0
> 00:00:21 ./memuser 768M 120 5 300
> 0 S root 1465 1 0 75 0 - 98683 schedu 10:20 pts/0
> 00:00:10 ./memuser 384M 300 10 600
> 0 S root 2148 1293 0 75 0 - 922 pipe_w 10:48 pts/0
> 00:00:00 grep -E memuser|kscand

The workload has been running for about half an hour, and kswapd cpu
usage doesn't seem significant. This is a 2GB guest running with my
patch ported to kvm.git HEAD. Guest is has 2G of memory.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.