On Sat, Jan 26, 2013 at 01:40:55PM +0900, Kyungmin Park wrote: > Hi, > > On 1/24/13, Simon Jeons <simon.jeons@xxxxxxxxx> wrote: > > Hi Minchan, > > On Wed, 2013-01-23 at 16:58 +0900, Minchan Kim wrote: > >> On Tue, Jan 22, 2013 at 02:53:41PM +0800, 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: > >> > >> Many of below item could be applied in in-memory swap like zram, zcache. > >> > >> > > >> > 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. > >> > >> Agreed. > >> > >> > 4. Swap map scan overhead. Swap in-memory map scan scans an array, which > >> > is > >> > very inefficient, especially if swap storage is fast. > >> > >> Agreed. > >> > > 5. SSD related optimization, mainly discard support. > > Now swap codes are based on each swap slots. it means it can't > optimize discard feature since getting meaningful performance gain, it > requires 2 pages at least. Of course it's based on eMMC. In case of > SSD. it requires more pages to support discard. > > To address issue. I consider the batched discard approach used at filesystem. > *Sometime* scan all empty slot and it issues discard continuous swap > slots as many as possible. I posted a patch to make discard async before, which is almost good to me, though we still discard a cluster. http://marc.info/?l=linux-mm&m=135087309208120&w=2 -- 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>