Re: [PATCH 5/8] mm: move lazily freed pages to inactive list

[Shaohua Li]

On Wed, Nov 04, 2015 at 09:53:42AM -0800, Shaohua Li wrote:
> On Tue, Nov 03, 2015 at 09:52:23AM +0900, Minchan Kim wrote:
> > On Fri, Oct 30, 2015 at 10:22:12AM -0700, Shaohua Li wrote:
> > > On Fri, Oct 30, 2015 at 04:01:41PM +0900, Minchan Kim wrote:
> > > > MADV_FREE is a hint that it's okay to discard pages if there is memory
> > > > pressure and we use reclaimers(ie, kswapd and direct reclaim) to free them
> > > > so there is no value keeping them in the active anonymous LRU so this
> > > > patch moves them to inactive LRU list's head.
> > > > 
> > > > This means that MADV_FREE-ed pages which were living on the inactive list
> > > > are reclaimed first because they are more likely to be cold rather than
> > > > recently active pages.
> > > > 
> > > > An arguable issue for the approach would be whether we should put the page
> > > > to the head or tail of the inactive list.  I chose head because the kernel
> > > > cannot make sure it's really cold or warm for every MADV_FREE usecase but
> > > > at least we know it's not *hot*, so landing of inactive head would be a
> > > > comprimise for various usecases.
> > > > 
> > > > This fixes suboptimal behavior of MADV_FREE when pages living on the
> > > > active list will sit there for a long time even under memory pressure
> > > > while the inactive list is reclaimed heavily.  This basically breaks the
> > > > whole purpose of using MADV_FREE to help the system to free memory which
> > > > is might not be used.
> > > 
> > > My main concern is the policy how we should treat the FREE pages. Moving it to
> > > inactive lru is definitionly a good start, I'm wondering if it's enough. The
> > > MADV_FREE increases memory pressure and cause unnecessary reclaim because of
> > > the lazy memory free. While MADV_FREE is intended to be a better replacement of
> > > MADV_DONTNEED, MADV_DONTNEED doesn't have the memory pressure issue as it free
> > > memory immediately. So I hope the MADV_FREE doesn't have impact on memory
> > > pressure too. I'm thinking of adding an extra lru list and wartermark for this
> > > to make sure FREE pages can be freed before system wide page reclaim. As you
> > > said, this is arguable, but I hope we can discuss about this issue more.
> > 
> > Yes, it's arguble. ;-)
> > 
> > It seems the divergence comes from MADV_FREE is *replacement* of MADV_DONTNEED.
> > But I don't think so. If we could discard MADV_FREEed page *anytime*, I agree
> > but it's not true because the page would be dirty state when VM want to reclaim. 
> 
> There certainly are other usage cases, but even your patch log mainly describes
> the jemalloc usage case, which uses MADV_DONTNEED.
> 
> > I'm also against with your's suggestion which let's discard FREEed page before
> > system wide page reclaim because system would have lots of clean cold page
> > caches or anonymous pages. In such case, reclaiming of them would be better.
> > Yeb, it's really workload-dependent so we might need some heuristic which is
> > normally what we want to avoid.
> > 
> > Having said that, I agree with you we could do better than the deactivation
> > and frankly speaking, I'm thinking of another LRU list(e.g. tentatively named
> > "ezreclaim LRU list"). What I have in mind is to age (anon|file|ez)
> > fairly. IOW, I want to percolate ez-LRU list reclaiming into get_scan_count.
> > When the MADV_FREE is called, we could move hinted pages from anon-LRU to
> > ez-LRU and then If VM find to not be able to discard a page in ez-LRU,
> > it could promote it to acive-anon-LRU which would be very natural aging
> > concept because it mean someone touches the page recenlty.
> > 
> > With that, I don't want to bias one side and don't want to add some knob for
> > tuning the heuristic but let's rely on common fair aging scheme of VM.
> > 
> > Another bonus with new LRU list is we could support MADV_FREE on swapless
> > system.
> > 
> > > 
> > > Or do you want to push this first and address the policy issue later?
> > 
> > I believe adding new LRU list would be controversial(ie, not trivial)
> > for maintainer POV even though code wouldn't be complicated.
> > So, I want to see problems in *real practice*, not any theoritical
> > test program before diving into that.
> > To see such voice of request, we should release the syscall.
> > So, I want to push this first.
> 
> The memory pressure issue isn't just in artificial test. In jemalloc, there is
> a knob (lg_dirty_mult) to control the rate memory should be purged (using
> MADV_DONTNEED). We already had several reports in our production environment
> changing the knob can cause extra memory usage (and swap and so on). If
> jemalloc uses MADV_FREE, jemalloc will not purge any memory, which is equivent
> to disable current MADV_DONTNEED (eg, lg_dirty_mult = -1). I'm sure this will
> cause the similar issue, eg (extram memory usage, swap). That said I don't
> object to push this first, but the memory pressue issue can happen in real
> production, I hope it's not ignored.

I think the question is if application uses MADV_DONTNEED originally, how much
better if we replace it to MADV_FREE compared to just delete the MADV_DONTNEED,
considering anonymous memory is hard to be reclaimed currently.

Thanks,
Shaohua
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