[LSF/MM TOPIC]swap improvements for fast SSD

[Shaohua Li <shli@kernel.org>]

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.

I had some progresses in these areas recently:
http://marc.info/?l=linux-mm&m=134665691021172&w=2
http://marc.info/?l=linux-mm&m=135336039115191&w=2
http://marc.info/?l=linux-mm&m=135882182225444&w=2
http://marc.info/?l=linux-mm&m=135754636926984&w=2
http://marc.info/?l=linux-mm&m=135754634526979&w=2
But a lot of problems remain. I'd like to discuss the issues at the meeting.

Thanks,
Shaohua

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