Hi Shaohua,
On 03/19/2013 01:57 PM, Shaohua Li wrote:
On Tue, Mar 19, 2013 at 09:32:39AM +0800, Simon Jeons wrote:
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?
Didn't look at hugetlb yet, but for THP, maybe it's an overkill to really do 2M
page swapping. My idea is to provide a special version of add_to_swap +
try_to_unmap in page reclaim. We still do huge page split, but in the split, we
also do 'unmap' to reduce unnecessary TLB flush. In the split, tail pages
should be added back to page_list of shrink_page_list() instead of lru list, so
tail pages can be pageout soon. In this way, we can use existing swap code (not
bothering changing arch code and swap space allocation for example) and reach
my goal (reduce tlb flush and improve IO pattern). But that said, I didn't do
any coding yet, this might be just wrong actually, but I'll try some time.
What will happen when swapin?
--
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>