On Wed, May 09 2007, Justin Piszcz wrote: > http://kerneltrap.org/node/8176 Oh > 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. > Also, when this patch is implemented for x86_64 and if merged into > mainline, what does this mean for performance? The sglist branch of block repo has x86-64 support now. I'll post a new patchset tomorrow. Performance wise, it's meant to help higher end hardware that need 2-4mb (or bigger) commands to get good performance. That also includes things like tapes that have big block sizes, getting a command of the right size there is the difference between good and abysmal performance. > I have an mdadm raid5 of 10 raptors and get 434MB/s write and 622MB/s > read, would I see an increase in performance with this patch? Perhaps, depends on a lot of factors. -- 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
