Re: [RFC] mm: remove swapcache page early

[Minchan Kim <minchan@kernel.org>]

Hi Dan,

On Wed, Mar 27, 2013 at 03:24:00PM -0700, Dan Magenheimer wrote:
> > From: Hugh Dickins [mailto:hughd@google.com]
> > Subject: Re: [RFC] mm: remove swapcache page early
> > 
> > On Wed, 27 Mar 2013, Minchan Kim wrote:
> > 
> > > Swap subsystem does lazy swap slot free with expecting the page
> > > would be swapped out again so we can't avoid unnecessary write.
> >                              so we can avoid unnecessary write.
> > >
> > > But the problem in in-memory swap is that it consumes memory space
> > > until vm_swap_full(ie, used half of all of swap device) condition
> > > meet. It could be bad if we use multiple swap device, small in-memory swap
> > > and big storage swap or in-memory swap alone.
> > 
> > That is a very good realization: it's surprising that none of us
> > thought of it before - no disrespect to you, well done, thank you.
> 
> Yes, my compliments also Minchan.  This problem has been thought of before
> but this patch is the first to identify a possible solution.

Thanks!

>  
> > And I guess swap readahead is utterly unhelpful in this case too.
> 
> Yes... as is any "swap writeahead".  Excuse my ignorance, but I
> think this is not done in the swap subsystem but instead the kernel
> assumes write-coalescing will be done in the block I/O subsystem,
> which means swap writeahead would affect zram but not zcache/zswap
> (since frontswap subverts the block I/O subsystem).

Frankly speaking, I don't know why you mentioned "swap writeahead"
in this point. Anyway, I dobut how it effect zram, too. A gain I can
have a mind is compress ratio would be high thorough multiple page
compression all at once.

> 
> However I think a swap-readahead solution would be helpful to
> zram as well as zcache/zswap.

Hmm, why? swap-readahead is just hint to reduce big stall time to
reduce on big seek overhead storage. But in-memory swap is no cost
for seeking. So unnecessary swap-readahead can make memory pressure
high and it could cause another page swap out so it could be swap-thrashing.
And for good swap-readahead hit ratio, swap device shouldn't be fragmented.
But as you know, there are many factor to prevent it in the kernel now
and Shaohua is tackling on it.

> 
> > > This patch changes vm_swap_full logic slightly so it could free
> > > swap slot early if the backed device is really fast.
> > > For it, I used SWP_SOLIDSTATE but It might be controversial.
> > 
> > But I strongly disagree with almost everything in your patch :)
> > I disagree with addressing it in vm_swap_full(), I disagree that
> > it can be addressed by device, I disagree that it has anything to
> > do with SWP_SOLIDSTATE.
> > 
> > This is not a problem with swapping to /dev/ram0 or to /dev/zram0,
> > is it?  In those cases, a fixed amount of memory has been set aside
> > for swap, and it works out just like with disk block devices.  The
> > memory set aside may be wasted, but that is accepted upfront.
> 
> It is (I believe) also a problem with swapping to ram.  Two
> copies of the same page are kept in memory in different places,
> right?  Fixed vs variable size is irrelevant I think.  Or am
> I misunderstanding something about swap-to-ram?
> 
> > Similarly, this is not a problem with swapping to SSD.  There might
> > or might not be other reasons for adjusting the vm_swap_full() logic
> > for SSD or generally, but those have nothing to do with this issue.
> 
> I think it is at least highly related.  The key issue is the
> tradeoff of the likelihood that the page will soon be read/written
> again while it is in swap cache vs the time/resource-usage necessary
> to "reconstitute" the page into swap cache.  Reconstituting from disk
> requires a LOT of elapsed time.  Reconstituting from
> an SSD likely takes much less time.  Reconstituting from
> zcache/zram takes thousands of CPU cycles.

Yeb. That's why I wanted to use SWP_SOLIDSTATE.

> 
> > The problem here is peculiar to frontswap, and the variably sized
> > memory behind it, isn't it?  We are accustomed to using swap to free
> > up memory by transferring its data to some other, cheaper but slower
> > resource.
> 
> Frontswap does make the problem more complex because some pages
> are in "fairly fast" storage (zcache, needs decompression) and
> some are on the actual (usually) rotating media.  Fortunately,
> differentiating between these two cases is just a table lookup
> (see frontswap_test).

Yeb, I thouht it could be a last resort because I'd like to avoid
lookup every swapin if possible.

> 
> > But in the case of frontswap and zmem (I'll say that to avoid thinking
> > through which backends are actually involved), it is not a cheaper and
> > slower resource, but the very same memory we are trying to save: swap
> > is stolen from the memory under reclaim, so any duplication becomes
> > counter-productive (if we ignore cpu compression/decompression costs:
> > I have no idea how fair it is to do so, but anyone who chooses zmem
> > is prepared to pay some cpu price for that).
> 
> Exactly.  There is some "robbing of Peter to pay Paul" and
> other complex resource tradeoffs.  Presumably, though, it is
> not "the very same memory we are trying to save" but a
> fraction of it, saving the same page of data more efficiently
> in memory, using less than a page, at some CPU cost.
> 
> > And because it's a frontswap thing, we cannot decide this by device:
> > frontswap may or may not stand in front of each device.  There is no
> > problem with swapcache duplicated on disk (until that area approaches
> > being full or fragmented), but at the higher level we cannot see what
> > is in zmem and what is on disk: we only want to free up the zmem dup.
> 
> I *think* frontswap_test(page) resolves this problem, as long as
> we have a specific page available to use as a parameter.

Agreed. Will do the method if we all agree on the way because there isn't
better approach.

> 
> > I believe the answer is for frontswap/zmem to invalidate the frontswap
> > copy of the page (to free up the compressed memory when possible) and
> > SetPageDirty on the PageUptodate PageSwapCache page when swapping in
> > (setting page dirty so nothing will later go to read it from the
> > unfreed location on backing swap disk, which was never written).
> 
> There are two duplication issues:  (1) When can the page be removed
> from the swap cache after a call to frontswap_store; and (2) When
> can the page be removed from the frontswap storage after it
> has been brought back into memory via frontswap_load.
> 
> This patch from Minchan addresses (1).  The issue you are raising

No. I am addressing (2).

> here is (2).  You may not know that (2) has recently been solved
> in frontswap, at least for zcache.  See frontswap_exclusive_gets_enabled.
> If this is enabled (and it is for zcache but not yet for zswap),
> what you suggest (SetPageDirty) is what happens.

I am blind on zcache so I didn't see it. Anyway, I'd like to address it
on zram and zswap.

> 
> > We cannot rely on freeing the swap itself, because in general there
> > may be multiple references to the swap, and we only satisfy the one
> > which has faulted.  It may or may not be a good idea to use rmap to
> > locate the other places to insert pte in place of swap entry, to
> > resolve them all at once; but we have chosen not to do so in the
> > past, and there's no need for that, if the zmem gets invalidated
> > and the swapcache page set dirty.
> 
> I see.  Minchan's patch handles the removal "reactively"... it
> might be possible to handle it more proactively.  Or it may
> be possible to take the number of references into account when
> deciding whether to frontswap_store the page as, presumably,
> the likelihood of needing to "reconstitute" the page sooner increases
> with each additional reference.
> 
> > Hugh
> 
> Very useful thoughts, Hugh.  Thanks much and looking forward
> to more discussion at LSF/MM!

Dan, Your thought is VERY useful. Thanks much and looking forward
to more discsussion at LFS/MM!

-- 
Kind regards,
Minchan Kim

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