On Thu, May 10 2007, Jan Engelhardt wrote: > On May 9 2007 15:38, Jens Axboe wrote: > >> I am a mdadm/disk/hard drive fanatic, I was curious: > >> > >> >On i386, we can at most fit 256 scatterlist elements into a page, > >> >and on x86-64 we are stuck with 128. So that puts us somewhere > >> >between 512kb and 1024kb for a single IO. > >> > >> How come 32bit is 256 and 64 is only 128? > >> > >> I am sure it is something very fundamental/simple but I was curious, I > >> would think x86_64 would fit/support more scatterlists in a page. > > > >Because of the size of the scatterlist structure. As pointers are bigger > >on 64-bit archs, the scatterlist structure ends up being bigger. The > >page size on x86-64 is 4kb, hence the number of structures you can fit > >in a page is smaller. > > I take it this problem "goes away" on arches with 8KB page_size? Not really, the 8kb page size just doubles the sg size. On a 64-bit arch, that would still only get you 1mb IO size. -- Jens Axboe - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
