* Jim Meyering wrote on Thu, May 28, 2009 at 09:33:21PM CEST: > Glen Lenker wrote: > > On Thu, Mar 26, 2009 at 09:50:08PM +0000, Ralf Wildenhues wrote: > >> > >> Example run, on an 8-way, and with cat'ed instances of the dictionary, > >> on tmpfs, timings best of three: > > > > Hey Ralf, did you happen to specify the amount of RAM sort should > > use. Not specifying enough RAM for sort would force break what would > > be a single internal sort into multiple internal sort passes and then > > an external sort. As it is external sort is still sequential.
No, I did not specify the amount of RAM. The system I tested on has plenty of RAM, way more than is needed for the sort. Specifying something like 2G of RAM does not make any visible difference. > > I ran these tests on a 256MB instance of the dictionary in tmpfs, on a > > 8-core machine specifying 2G of RAM. > > > > runtime [s] threads > > file size [MB] 1 2 4 8 > > 256 2m41.219 1m27.357 52.53 36.429 > > 160.22user 1.34system 2:41.61elapsed 99%CPU > > 159.83user 1.45system 1:27.12elapsed 185%CPU > > 159.84user 1.56system 0:52.26elapsed 308%CPU > > 160.67user 1.53system 0:36.26elapsed 447%CPU > > > > This seems to be what I would expect from a good implementation. Yes, 56% efficiency going from 1 to 8 threads sounds like a much better number, also your system overhead looks very sane compared to what I saw. Seems like it's a system-specific issue after all. Which Linux kernel version and which pthread (glibc) version are you using? > >> It suggests to me that too much time is spent busy-waiting in pthread_join, > >> or that sort is computing too much (I haven't looked at the patch in > >> detail). > >> > >> Also, I'd have expected the rate going from 1 to 2 threads to get at least > >> a bit better with bigger file size, but it remains remarkably constant, > >> around 1.45 for this setup. What am I missing? Glen, could you look at this, too? I mean just timings of 1 vs 2 threads for different file sizes? Thanks. > I ran some tests (see below), and got results that look > similar to Ralf's. I used an otherwise-idle 16-core Nehalem-based > system with 8GB of RAM. Which kernel and glibc? > However, more importantly, while the 16- and 8-thread tests were > running, I sampled thread counts using ps and was surprised to see > the number of active threads was usually far less than the maximum. Well yes, I suppose you are seeing the serial overhead from reading in the data set by the first process alone, and from the later merging stages where only fewer threads are active. > T=16 T is number of threads > 12.96 <--- This is elapsed wall-clock time in seconds. > 16.43 > 13.76 > 16.60 That varies a lot. Are there other jobs on this system running? Do you, Jim, also see high system time overhead? That would support the hypothesis of a less efficient thread implementation (or kernel issue) being the cause. Off-list, Glen wrote: > I don't know if this is related, because up until now memory > consumption hasn't been mentioned wrt to this patch, but this patch > bumps the amount of memory consumed from 1.5x to 2x. Should we > consider reducing this? I don't think fighting over that 0.5 x size is worth any hassles, specifically as that x is just the number of lines times the size of a struct line, not the size of the data in one line (right?). Cheers, Ralf _______________________________________________ Bug-coreutils mailing list [email protected] http://lists.gnu.org/mailman/listinfo/bug-coreutils
