On 09/03/2012 03:02 PM, Konstantin Khlebnikov wrote:
Shaohua Li wrote:On Mon, Sep 03, 2012 at 05:32:45PM +0900, Minchan Kim wrote: Subject: swap: add a simple random read swapin detection The swapin readahead does a blind readahead regardless if the swapin is sequential. This is ok for harddisk and random read, because read big size has no penality in harddisk, and if the readahead pages are garbage, they can be reclaimed fastly. But for SSD, big size read is more expensive than small size read. If readahead pages are garbage, such readahead only has overhead. This patch addes a simple random read detection like what file mmap readahead does. If random read is detected, swapin readahead will be skipped. This improves a lot for a swap workload with random IO in a fast SSD. I run anonymous mmap write micro benchmark, which will triger swapin/swapout. runtime changes with path randwrite harddisk -38.7% seqwrite harddisk -1.1% randwrite SSD -46.9% seqwrite SSD +0.3% For both harddisk and SSD, the randwrite swap workload run time is reduced significant. sequential write swap workload hasn't chanage. Interesting is the randwrite harddisk test is improved too. This might be because swapin readahead need allocate extra memory, which further tights memory pressure, so more swapout/swapin.Generally speaking swapin readahread isn't usable while system is under memory pressure. Cache hit isn't very probable, because reclaimer allocates swap entries in page-LRU order. But swapin readahead is very useful if system recovers from memory pressure, it helps to read whole swap back to memory (a sort of desktop scenario). So, I think we can simply disable swapin readahead while system is under memory pressure. For example in time-based manner -- enable it only after grace period after last swap_writepage().
Determining "under memory pressure" is pretty hard to do. However, Shaohua's patch provides an easy way to see whether swap readahead is helping (we are getting pages from the swap cache), or whether it is not (pages got evicted before someone faulted on them). In short, Shaohua's patch not only does roughly what you want, it does it in a simple way. -- All rights reversed -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>