On Thu, Apr 24, 2014 at 09:58:11AM -0700, Dave Hansen wrote: > On 04/24/2014 01:45 AM, Mel Gorman wrote: > >> +/* > >> + * See Documentation/x86/tlb.txt for details. We choose 33 > >> + * because it is large enough to cover the vast majority (at > >> + * least 95%) of allocations, and is small enough that we are > >> + * confident it will not cause too much overhead. Each single > >> + * flush is about 100 cycles, so this caps the maximum overhead > >> + * at _about_ 3,000 cycles. > >> + */ > >> +/* in units of pages */ > >> +unsigned long tlb_single_page_flush_ceiling = 1; > >> + > > > > This comment is premature. The documentation file does not exist yet and > > 33 means nothing yet. Out of curiousity though, how confident are you > > that a TLB flush is generally 100 cycles across different generations > > and manufacturers of CPUs? I'm not suggesting you change it or auto-tune > > it, am just curious. > > Yeah, the comment belongs in the later patch where I set it to 33. > > I looked at this on the last few generations of Intel CPUs. "100 > cycles" was a very general statement, and not precise at all. My laptop > averages out to 113 cycles overall, but the flushes of 25 pages averaged > 96 cycles/page while the flushes of 2 averaged 219/page. > > Those cycles include some costs of from the instrumentation as well. > > I did not test on other CPU manufacturers, but this should be pretty > easy to reproduce. I'm happy to help folks re-run it on other hardware. > > I also believe with the modalias stuff we've got in sysfs for the CPU > objects we can do this in the future with udev rules instead of > hard-coding it in the kernel. > You convinced me. Regardless of whether you move the comment or update the changelog; Acked-by: Mel Gorman <mgorman@xxxxxxx> -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>