Thanks, Ross. You basically say that while the pmemd.cuda CPU process is
waiting for I/O, it has 100 % CPU usage. I am wondering about this, just
out of curiosity: normally, if a process is waiting for I/O (from/to
sockets, filesystem, ...), it is kept idle and waiting for a signal to
handle. Is such an event-driven architechture not possible for
GPU-CPU-communication?
All the best,
Jan-Philip
On 01/11/2012 04:03 PM, Ross Walker wrote:
> Hi Jan-Philip,
>
>> you've shown a significant change of "double GPU" performance in
>> dependence of CPU performance.
>>
>> I am wondering if this dependency can also exist for "single GPU"
>> simulations. How can I find out if my CPU is limiting the performance
>> of
>> a GPU simulation without trying different CPUs? Is the CPU usage an
>> indicator? I've been running a simulation of a small system on a single
>> C2070 and saw the pmemd process creating a CPU usage of 100 %. Do these
>> 100 % already suggest that the CPU limits the performance?
>
> The way pmemd.cuda is written is such that for serial GPU calculations the
> entire calculation is run on the GPU, the CPU does nothing except sit
> spinning at barriers waiting to do I/O. As such the CPU speed and memory
> bandwidth (and to some extent the speed of the PCI-E slot) will have little
> to no impact on the GPU code performance for a single GPU run.
>
> The CPU is at 100% utilization because it is basically sitting there
> spinning waiting for interrupts hence this doesn't mean anything with
> regards to performance.
>
> In parallel (using more than one GPU for a single run) the situation is very
> different. Here the GPUs have to communicate on every time step. Hence the
> PCI-E connection speed needs to be at least x16 PER GPU and the higher the
> memory bandwidth of the CPU memory (related to the front side bus speed of
> the CPU) the better.
>
> All the best
> Ross
>
> /\
> \/
> |\oss Walker
>
> ---------------------------------------------------------
> | Assistant Research Professor |
> | San Diego Supercomputer Center |
> | Adjunct Assistant Professor |
> | Dept. of Chemistry and Biochemistry |
> | University of California San Diego |
> | NVIDIA Fellow |
> | http://www.rosswalker.co.uk | http://www.wmd-lab.org/ |
> | Tel: +1 858 822 0854 | EMail:- ross.rosswalker.co.uk |
> ---------------------------------------------------------
>
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Received on Wed Jan 11 2012 - 08:00:02 PST
