On Tue, Oct 16, 2007 at 05:34:15PM +1000, Nick Piggin wrote: > > It's a hard call. The I/O time for 1MB of contiguous disk data > > is about the I/O time of 512 bytes of contiguous disk data. > > And if you're thrashing, then by definition you need to throw > out 1MB of your working set in order to read it in. Right. But you need a differential hit rate of only a few percent on that 1020 extra kb of data you swapped in versus the 1Mb of data you swapped out for this to be advantageous. With "differential hit rate" I mean the chances of getting a hit on the 1Mb of data just paged in, minus the chances of getting a hit on the 1Mb of data just paged out. With a little luck that 1Mb that is paged out didn't get used for quite a while, while there is a hint that the 1Mb you're paging in is active, as one of its sub-pages just got a hit. So... IMHO, it would be useful to implement something that pages out chunks of memory larger than a single hardware page. This would reduce the size of the memory management tables (*), as well as improve disk throughput if things DO come to paging.... This should of course be configurable. Some workloads are better off with a virtual page size of 8k, some with 128k. some with 1M. As far as I can see, the "page-cluster" parameter defines how many pages at a time are selected for page-out at a time. This increases the page-out efficiency. Improving the page-in efficiency is also useful: It is the other half of hte equation. Roger. (*) If the kernel starts working with a 1Mb virtual page size, you need a 256 times smaller mapping table between processes and memory or swap. Of course, the hardware doesn't support this (actually, it does for 1Mb virtual pages), so you'll have to create 256 page table entries for the hardware instead of just one. -- ** R.E.Wolff@xxxxxxxxxxxx ** http://www.BitWizard.nl/ ** +31-15-2600998 ** ** Delftechpark 26 2628 XH Delft, The Netherlands. KVK: 27239233 ** *-- BitWizard writes Linux device drivers for any device you may have! --* Q: It doesn't work. A: Look buddy, doesn't work is an ambiguous statement. Does it sit on the couch all day? Is it unemployed? Please be specific! Define 'it' and what it isn't doing. --------- Adapted from lxrbot FAQ - To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
