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

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

Hi Shaohua,
On 03/19/2013 09:27 AM, Shaohua Li wrote:
> 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.
Do you have any idea about implement this feature?


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