Assuming that the penalty for page faults reduces ideally with the number of threads, one would obtain an average ~4us with 240 active threads. Nevertheless, 850 us is terribly large, so Amdahl's Law will hit hard...
Since I don't think the Xeon Phi will be the last type of accelerator from Intel, why not include such a test? I don't know how portable this is, though... Best regards, Karli On 12/17/2012 11:23 AM, Jed Brown wrote: > He should be using all the threads for this. Using one thread to fault a > bunch of memory is a recipe for terrible performance all around. > > > On Mon, Dec 17, 2012 at 9:21 AM, Barry Smith <bsmith at mcs.anl.gov > <mailto:bsmith at mcs.anl.gov>> wrote: > > > Should we have a test like this in the PETSc benchmark directory? > > Barry > > > Begin forwarded message: > > > From: Kazutomo Yoshii <kazutomo at mcs.anl.gov > <mailto:kazutomo at mcs.anl.gov>> > > Subject: [Xlab] slow pagefault on KNC > > Date: December 17, 2012 10:48:06 AM CST > > To: "xlab at cels.anl.gov <mailto:xlab at cels.anl.gov>" > <xlab at cels.anl.gov <mailto:xlab at cels.anl.gov>> > > > > Hi, > > > > I noticed that pagefault is very slow on KNC. It takes 850 usec > > while it takes ~1 usec on Xeon, so prefaulting 1GB of memory region > > takes 222 sec on KNC. > > > > This may not impact a big app runs for hours and hours, but I guess > > this definitely affects short-lived processes or threads, which might > > make MIC less fascinating. > > > > This could be a hardware problem(need to check Phi), a kernel bug, > > or maybe sage of SMP kernel on many-core. > > If this is the last case, it would be really interesting for me. > > > > Attached a simple page fault benchmark. > > > > - kaz > [see attached file: > pftest.c]_______________________________________________ > > Xlab mailing list > > Xlab at lists.cels.anl.gov <mailto:Xlab at lists.cels.anl.gov> > > https://lists.cels.anl.gov/mailman/listinfo/xlab > >
