On Mon, Mar 18, 2013 at 06:38:29PM +0800, Bob Liu wrote: > > On 03/15/2013 05:39 PM, Simon Jeons wrote: > > On 01/22/2013 02:53 PM, Shaohua Li wrote: > >> Hi, > >> > >> Because of high density, low power and low price, flash storage (SSD) > >> is a good > >> candidate to partially replace DRAM. A quick answer for this is using > >> SSD as > >> swap. But Linux swap is designed for slow hard disk storage. There are > >> a lot of > >> challenges to efficiently use SSD for swap: > >> > >> 1. Lock contentions (swap_lock, anon_vma mutex, swap address space lock) > >> 2. TLB flush overhead. To reclaim one page, we need at least 2 TLB > >> flush. This > >> overhead is very high even in a normal 2-socket machine. > >> 3. Better swap IO pattern. Both direct and kswapd page reclaim can do > >> swap, > >> which makes swap IO pattern is interleave. Block layer isn't always > >> efficient > >> to do request merge. Such IO pattern also makes swap prefetch hard. > >> 4. Swap map scan overhead. Swap in-memory map scan scans an array, > >> which is > >> very inefficient, especially if swap storage is fast. > >> 5. SSD related optimization, mainly discard support > >> 6. Better swap prefetch algorithm. Besides item 3, sequentially > >> accessed pages > >> aren't always in LRU list adjacently, so page reclaim will not swap > >> such pages > >> in adjacent storage sectors. This makes swap prefetch hard. > >> 7. Alternative page reclaim policy to bias reclaiming anonymous page. > >> Currently reclaim anonymous page is considering harder than reclaim > >> file pages, > >> so we bias reclaiming file pages. If there are high speed swap > >> storage, we are > >> considering doing swap more aggressively. > >> 8. Huge page swap. Huge page swap can solve a lot of problems above, > >> but both > >> THP and hugetlbfs don't support swap. > > > > Could you tell me in which workload hugetlb/thp pages can't swapout > > influence your performance? Is it worth? > > > > I'm also very interesting in this workload. > I think hugetlb/thp pages can be a potential user of zprojects like > zswap/zcache. > We can try to compress those pages before breaking them to normal pages. I don't have particular workload and don't have data for obvious reason. What I expected is swapout hugetlb/thp is to reduce some overheads (eg, tlb flush) and improve IO pattern. -- 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>