Re: [LSF/MM TOPIC]swap improvements for fast SSD

[Simon Jeons <simon.jeons@gmail.com>]

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 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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