Re: [RFC 00/11] THP swap: Delay splitting THP during swapping out

[Minchan Kim]

On Thu, Aug 18, 2016 at 08:44:13PM -0700, Huang, Ying wrote:
> Minchan Kim <minchan@xxxxxxxxxx> writes:
> 
> > Hi Huang,
> >
> > On Thu, Aug 18, 2016 at 10:19:32AM -0700, Huang, Ying wrote:
> >> Minchan Kim <minchan@xxxxxxxxxx> writes:
> >> 
> >> > Hi Tim,
> >> >
> >> > On Wed, Aug 17, 2016 at 10:24:56AM -0700, Tim Chen wrote:
> >> >> On Wed, 2016-08-17 at 14:07 +0900, Minchan Kim wrote:
> >> >> > On Tue, Aug 16, 2016 at 07:06:00PM -0700, Huang, Ying wrote:
> >> >> > > 
> >> >> > >
> >> >> > > > 
> >> >> > > > I think Tim and me discussed about that a few weeks ago.
> >> >> > > I work closely with Tim on swap optimization.?This patchset is the part
> >> >> > > of our swap optimization plan.
> >> >> > > 
> >> >> > > > 
> >> >> > > > Please search below topics.
> >> >> > > > 
> >> >> > > > [1] mm: Batch page reclamation under shink_page_list
> >> >> > > > [2] mm: Cleanup - Reorganize the shrink_page_list code into smaller functions
> >> >> > > > 
> >> >> > > > It's different with yours which focused on THP swapping while the suggestion
> >> >> > > > would be more general if we can do so it's worth to try it, I think.
> >> >> > > I think the general optimization above will benefit both normal pages
> >> >> > > and THP at least for now.?And I think there are no hard conflict
> >> >> > > between those two patchsets.
> >> >> > If we could do general optimzation, I guess THP swap without splitting
> >> >> > would be more straight forward.
> >> >> > 
> >> >> > If we can reclaim batch a certain of pages all at once, it helps we can
> >> >> > do scan_swap_map(si, SWAP_HAS_CACHE, nr_pages). The nr_pages could be
> >> >> > greater or less than 512 pages. With that, scan_swap_map effectively
> >> >> > search empty swap slots from scan_map or free cluser list.
> >> >> > Then, needed part from your patchset is to just delay splitting of THP.
> >> >> > 
> >> >> > > 
> >> >> > > 
> >> >> > > The THP swap has more opportunity to be optimized, because we can batch
> >> >> > > 512 operations together more easily.?For full THP swap support, unmap a
> >> >> > > THP could be more efficient with only one swap count operation instead
> >> >> > > of 512, so do many other operations, such as add/remove from swap cache
> >> >> > > with multi-order radix tree etc.?And it will help memory fragmentation.
> >> >> > > THP can be kept after swapping out/in, need not to rebuild THP via
> >> >> > > khugepaged.
> >> >> > It seems you increased cluster size to 512 and search a empty cluster
> >> >> > for a THP swap. With that approach, I have a concern that once clusters
> >> >> > will be fragmented, THP swap support doesn't take benefit at all.
> >> >> > 
> >> >> > Why do we need a empty cluster for swapping out 512 pages?
> >> >> > IOW, below case could work for the goal.
> >> >> > 
> >> >> > A : Allocated slot
> >> >> > F : Free slot
> >> >> > 
> >> >> > cluster A?cluster B
> >> >> > AAAAFFFF?-?FFFFAAAA
> >> >> > 
> >> >> > That's one of the reason I suggested batch reclaim work first and
> >> >> > support THP swap based on it. With that, scan_swap_map can be aware of nr_pages
> >> >> > and selects right clusters.
> >> >> > 
> >> >> > With the approach, justfication of THP swap support would be easier, too.
> >> >> > IOW, I'm not sure how only THP swap support is valuable in real workload.
> >> >> > 
> >> >> > Anyways, that's just my two cents.
> >> >> 
> >> >> Minchan,
> >> >> 
> >> >> Scanning for contiguous slots that span clusters may take quite a
> >> >> long time under fragmentation, and may eventually fail. In that case the addition scan
> >> >> time overhead may go to waste and defeat the purpose of fast swapping of large page.
> >> >> 
> >> >> The empty cluster lookup on the other hand is very fast.
> >> >> We treat the empty cluster available case as an opportunity for fast path
> >> >> swap out of large page. Otherwise, we'll revert to the current
> >> >> slow path behavior of breaking into normal pages so there's no
> >> >> regression, and we may get speed up. We can be considerably faster when a lot of large
> >> >> pages are used. 
> >> >
> >> > I didn't mean we should search scan_swap_map firstly without peeking
> >> > free cluster but what I wanted was we might abstract it into
> >> > scan_swap_map.
> >> >
> >> > For example, if nr_pages is greather than the size of cluster, we can
> >> > get empty cluster first and nr_pages - sizeof(cluster) for other free
> >> > cluster or scanning of current CPU per-cpu cluster. If we cannot find
> >> > used slot during scanning, we can bail out simply. Then, although we
> >> > fail to get all* contiguous slots, we get a certain of contiguous slots
> >> > so it would be benefit for seq write and lock batching point of view
> >> > at the cost of a little scanning. And it's not specific to THP algorighm.
> >> 
> >> Firstly, if my understanding were correct, to batch the normal pages
> >> swapping out, the swap slots need not to be continuous.  But for the THP
> >> swap support, we need the continuous swap slots.  So I think the
> >> requirements are quite different between them.
> >
> > Hmm, I don't understand.
> >
> > Let's think about swap slot management layer point of view.
> > It doesn't need to take care of that a amount of batch request is caused
> > by a thp page or multiple normal pages.
> >
> > A matter is just that VM now asks multiple swap slots for seveal LRU-order
> > pages so swap slot management tries to allocate several slots in a lock.
> > Sure, it would be great if slots are consecutive fully because it means
> > it's fast big sequential write as well as readahead together ideally.
> > However, it would be better even if we didn't get consecutive slots because
> > we get muliple slots all at once by batch.
> >
> > It's not a THP specific requirement, I think.
> > Currenlty, SWAP_CLUSTER_MAX might be too small to get a benefit by
> > normal page batch but it could be changed later once we implement batching
> > logic nicely.
> 
> Consecutive or not may influence the performance of the swap slots
> allocation function greatly.  For example, there is some non-consecutive
> swap slots at the begin of the swap space, and some consecutive swap
> slots at the end of the swap space.  If the consecutive swap slots are
> needed, the function may need to scan from the begin to the end.  If
> non-consecutive swap slots are required, just return the swap slots at
> the begin of the swap space.

Don't get me wrong. I never said consecutive swap slot allocation is
not important and should scan swap_map fully for searching consecutive
swap slot.

Both multiple normal page swap and a THP swap, consecutive swap slot
allocation is important so that it's a same requirement so I want to
abstract it regardless of THP swap.

> 
> >> And with the current design of the swap space management, it is quite
> >> hard to implement allocating nr_pages continuous free swap slots.  To
> >> reduce the contention of sis->lock, even to scan one free swap slot, the
> >> sis->lock is unlocked during scanning.  When we scan nr_pages free swap
> >> slots, and there are no nr_pages continuous free swap slots, we need to
> >> scan from sis->lowest_bit to sis->highest_bit, and record the largest
> >> continuous free swap slots.  But when we lock sis->lock again to check,
> >> some swap slot inside the largest continuous free swap slots we found
> >> may be allocated by other processes.  So we may end up with a much
> >> smaller number of swap slots or we need to startover again.  So I think
> >> the simpler solution is to
> >> 
> >> - When a whole cluster is requested (for the THP), try to allocate a
> >>   free cluster.  Give up if there are no free clusters.
> >
> > One thing I'm afraid that it would consume free clusters very fast
> > if adjacent pages around a faulted one doesn't have same hottness/
> > lifetime. Once it happens, we can't get benefit any more.
> > IMO, it's too conservative and might be worse for the fragment point
> > of view.
> 
> It is possible.  But I think we should start from the simple solution
> firstly.  Instead of jumping to the perfect solution directly.
> Especially when the simple solution is a subset of the perfect solution.
> Do you agree?

If simple solution works well and is hard to prove it's not bad than as-is,
I agree. But my concern is about that it would consume free clusters so fast
that it can affect badly for other workload.

> 
> There are some other difficulties not to use the swap cluster to hold
> the THP swapped out for the full THP swap support (without splitting).
> 
> The THP could be mapped in both PMD and PTE.  After the THP is swapped
> out.  There may be swap entry in PMD and PTE too.  If a non-head PTE is
> accessed, how do we know where is the first swap slot for the THP, so
> that we can swap in the whole THP?

You mean you want to swapin 2M pages all at once? Hmm, I'm not sure
it's a good idea. We don't have any evidence 512 pages have same time
locality. They were just LRU in-order due to split implementation,
not time locality. A thing we can bet is any processes sharing the THP
doesn't touch a subpage in 512 pages so it's really *cold*.
For such cold 512 page swap-in, I am really not sure.

> 
> We can have a flag in cluster_info->flag to mark whether the swap
> cluster backing a THP.  So swap in readahead can avoid to read ahead the
> THP, or it can read ahead the whole THP instead of just several
> sub-pages of the THP.
> 
> And if we use one swap cluster for each THP, we can use cluster_info->data
> to hold compound map number.  That is very convenient.

Huang,

If you think my points are enough valid, just continue your work
regardless of my comment. I don't want to waste your time if it helps
your workload really. And I will defer the decision to other MM people.

What I just wanted is to make swap batch for normal pages first and
then support THP swap based upon it because normal page batching would
more general optimization for us and I thought it will make your work
more simple.

Thanks.