On Tue, 11 September 2007 04:52:19 +1000, Nick Piggin wrote: > On Tuesday 11 September 2007 16:03, Christoph Lameter wrote: > > > 5. VM scalability > > Large block sizes mean less state keeping for the information being > > transferred. For a 1TB file one needs to handle 256 million page > > structs in the VM if one uses 4k page size. A 64k page size reduces > > that amount to 16 million. If the limitation in existing filesystems > > are removed then even higher reductions become possible. For very > > large files like that a page size of 2 MB may be beneficial which > > will reduce the number of page struct to handle to 512k. The variable > > nature of the block size means that the size can be tuned at file > > system creation time for the anticipated needs on a volume. > > The idea that there even _is_ a bug to fail when higher order pages > cannot be allocated was also brushed aside by some people at the > vm/fs summit. I don't know if those people had gone through the > math about this, but it goes somewhat like this: if you use a 64K > page size, you can "run out of memory" with 93% of your pages free. > If you use a 2MB page size, you can fail with 99.8% of your pages > still free. That's 64GB of memory used on a 32TB Altix. While I agree with your concern, those numbers are quite silly. The chances of 99.8% of pages being free and the remaining 0.2% being perfectly spread across all 2MB large_pages are lower than those of SHA1 creating a collision. I don't see anyone abandoning git or rsync, so your extreme example clearly is the wrong one. Again, I agree with your concern, even though your example makes it look silly. Jörn -- You can't tell where a program is going to spend its time. Bottlenecks occur in surprising places, so don't try to second guess and put in a speed hack until you've proven that's where the bottleneck is. -- Rob Pike - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
