Messages in this thread | | | Date | Wed, 27 May 2009 03:26:58 +0200 | From | Johannes Weiner <> | Subject | Re: [RFC][PATCH 5/5] (experimental) chase and free cache only swap |
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On Wed, May 27, 2009 at 09:08:13AM +0900, KAMEZAWA Hiroyuki wrote: > On Tue, 26 May 2009 20:14:00 +0200 > Johannes Weiner <hannes@cmpxchg.org> wrote: > > > On Tue, May 26, 2009 at 12:18:34PM +0900, KAMEZAWA Hiroyuki wrote: > > > > > > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> > > > > > > Just a trial/example patch. > > > I'd like to consider more. Better implementation idea is welcome. > > > > > > When the system does swap-in/swap-out repeatedly, there are > > > cache-only swaps in general. > > > Typically, > > > - swapped out in past but on memory now while vm_swap_full() returns true > > > pages are cache-only swaps. (swap_map has no references.) > > > > > > This cache-only swaps can be an obstacles for smooth page reclaiming. > > > Current implemantation is very naive, just scan & free. > > > > I think we can just remove that vm_swap_full() check in do_swap_page() > > and try to remove the page from swap cache unconditionally. > > > I'm not sure why reclaim swap entry only at write fault.
How do you come to that conclusion? Do you mean the current code does that? Did you understand that I suggested that?
> > If it's still mapped someplace else, we let it cached. If not, there > > is not much use for keeping it around and we free it. > > > yes. > > > When I removed it and did benchmarks, I couldn't spot any difference > > in the timings, though. Did you measure the benefits of your patch > > somehow? > My patche has no "performance benefit". (My patch description may be bad.) > I just checked that cache-only-swap can be big.(by sysrq-m) > > Even when we remove vm_swap_full() in do_swap_page(), > swapin-readahead + trylock-at-zap + vmscan makes "unused" swap caches easily. > It reaches 1M in 2hours test of heavy swap program.
Ouch.
> > According to the git history tree, vm_swap_full() was initially only > > used to aggressively drop cache entries even when they are mapped. > > > > Rik put it into vmscan to reclaim swap cache _at all_ for activated > > pages. But I think unconditionally dropping the cache entry makes > > sense if the page gets shuffled around on the LRU list. Better to > > re-allocate a swap slot close to the new LRU buddies on the next scan. > > > > And having this all covered, the need for the scanning your patch does > > should be gone, unless I missed something. > > > Considering memcg, global lru scanning is no help ;( > And I'm writing this patch for memcg.
Oh, sorry. That makes sense of course.
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