Am 21.03.2014 06:35, schrieb Stefan Zager: > On Thu, Mar 20, 2014 at 10:21 PM, Duy Nguyen <pclo...@gmail.com> wrote: >> On Fri, Mar 21, 2014 at 08:51:18AM +0700, Duy Nguyen wrote: >>> On Thu, Mar 20, 2014 at 11:08 PM, Stefan Zager <sza...@chromium.org> wrote: >>>> Duy, would you like to re-post your patch without the new pread >>>> implementation? >>> >>> I will but let me try out the sliding window idea first. My quick >>> tests on git.git show me we may only need 21k mmap instead of 177k >>> pread. That hints some potential performance improvement. >> >> The patch at the bottom reuses (un)use_pack() instead of pread(). The >> results on linux-2.6 do not look any different. I guess we can drop >> the idea. >> >> It makes me wonder, though, what's wrong a simple patch like this to >> make pread in index-pack thread-safe? It does not look any different >> either from the performance point of view, perhaps because >> unpack_data() reads small deltas most of the time > > When you serialize disk access in this way, the effect on performance > is really dependent on the behavior of the OS, as well as the locality > of the read offsets. Assuming -- fairly, I think -- that the reads > will be pretty randomly distributed (i.e., no locality to speak of), > then your best bet is get as many read operations in flight as > possible, and let the disk scheduler optimize the seek time. >
The read() implementation in MSVCRT.DLL is synchronized anyway, and I strongly suspect that this is also true for ReadFile() (at least for synchronous file handles, i.e. opened without FILE_FLAG_OVERLAPPED). So I guess separate file descriptors would help with parallel IO as well. -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
