Re: [Valgrind-users] Performance profiling in VM using Valgrind

[Josef W. <Jos...@gm...>]

Am 24.04.2013 05:44, schrieb Liu James:
> Dear all,
>
> I used Valgrind(callgrind) to profile database cache performance, and
> the host machine is a VM guest with XEON CPU.
>
> Without instrumentation, the performance of dbms with cache-on is 20%
>   higher

What do you mean by higher? Better or worse performance?

  than cache-off. With callgrind, the call graph shows there is a
> 5x performance improvement.

For Callgrid/Cachegrind, the environment doesn't matter as it's a
simulation of a simple cache hierarchy, only takeing user level into
account. So it does not matter if it's running within a VM guest.

BUT:
I would assume that user-level simulation can not really capture
what's going on in a database, as there probably is much I/O and
the OS involved, and more important, the performance probably depends
on external latency/bandwidth contrains from network/drives etc.

> My point is whether Valgrind is suitable for profiling working in VM?
>   e.g, with IR conversion, the performance gap becomes larger, which
> does not reflect the real gap.   If you are familiar with profiling, I
> appreciate you could recommend some other profile tools.

Any profiler which does system-wide sampling should be fine (perf, 
oprofile, other Intel/AMD tools, ...). But the VM must support to
pass performance counters to guests, if that tool should work in the
guest. I think that perf running within a KVM guest should be able to
do this.
Interpreting the results is another issue. An idea would be to
check how much the performance depends on external constrains. A VM
should allow to throttle e.g. the network connection.

On the other hand - as John suggested - why not get rid of the VM for
profiling first?

More tools should work, and you also can compare results with/without
VM afterwards. The VM may play a role if a lot of TLB misses are
involved - even with hardware support (extended/nested page tables)
a TLB miss may be way slower in a VM guest.

Josef

>
>
> Thanks a lot!!!
>
>
> Best regards,
> James
>
>
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