> From: Minchan Kim [mailto:minchan@xxxxxxxxxx] > Subject: Re: zsmalloc defrag (Was: [PATCH] mm: remove compressed copy from zram in-memory) > > On Tue, Apr 09, 2013 at 01:25:45PM -0700, Dan Magenheimer wrote: > > > From: Minchan Kim [mailto:minchan@xxxxxxxxxx] > > > Subject: Re: zsmalloc defrag (Was: [PATCH] mm: remove compressed copy from zram in-memory) > > > > > > Hi Dan, > > > > > > On Mon, Apr 08, 2013 at 09:32:38AM -0700, Dan Magenheimer wrote: > > > > > From: Minchan Kim [mailto:minchan@xxxxxxxxxx] > > > > > Sent: Monday, April 08, 2013 12:01 AM > > > > > Subject: [PATCH] mm: remove compressed copy from zram in-memory > > > > > > > > (patch removed) > > > > > > > > > Fragment ratio is almost same but memory consumption and compile time > > > > > is better. I am working to add defragment function of zsmalloc. > > > > > > > > Hi Minchan -- > > > > > > > > I would be very interested in your design thoughts on > > > > how you plan to add defragmentation for zsmalloc. In > > > > > > What I can say now about is only just a word "Compaction". > > > As you know, zsmalloc has a transparent handle so we can do whatever > > > under user. Of course, there is a tradeoff between performance > > > and memory efficiency. I'm biased to latter for embedded usecase. > > > > Have you designed or implemented this yet? I have a couple > > of concerns: > > Not yet implemented but just had a time to think about it, simply. > So surely, there are some obstacle so I want to uncase the code and > number after I make a prototype/test the performance. > Of course, if it has a severe problem, will drop it without wasting > many guys's time. OK. I have some ideas that may similar or may be very different than yours. Likely different, since I am coming at it from the angle of zcache which has some different requirements. So I'm hoping that by discussing design we can incorporate some of the zcache requirements before coding. > > 1) The handle is transparent to the "user", but it is still a form > > of a "pointer" to a zpage. Are you planning on walking zram's > > tables and changing those pointers? That may be OK for zram > > but for more complex data structures than tables (as in zswap > > and zcache) it may not be as easy, due to races, or as efficient > > because you will have to walk potentially very large trees. > > Rough concept is following as. > > I'm considering for zsmalloc to return transparent fake handle > but we have to maintain it with real one. > It could be done in zsmalloc internal so there isn't any race we should consider. That sounds very difficult because I think you will need an extra level of indirection to translate every fake handle to every real handle/pointer (like virtual-to-physical page tables). Or do you have some more clever idea? > > 2) Compaction in the kernel is heavily dependent on page migration > > and page migration is dependent on using flags in the struct page. > > There's a lot of code in those two code modules and there > > are going to be a lot of implementation differences between > > compacting pages vs compacting zpages. > > Compaction of kernel is never related to zsmalloc's one. OK. Compaction has certain meaning in the kernel. Defrag is usually used I think for what we are discussing here. So I thought you might be planning on doing exactly what the kernel does that it calls compaction. > > I'm also wondering if you will be implementing "variable length > > zspages". Without that, I'm not sure compaction will help > > enough. (And that is a good example of the difference between > > Why do you think so? > variable lengh zspage could be further step to improve but it's not > only a solution to solve fragmentation. In my partial-design-in-my-head, they are related, but I think I understand what you mean. You are planning to move zpages across zspage boundaries, and I am not. So I think your solution will result in better density but may be harder to implement. > > > > particular, I am wondering if your design will also > > > > handle the requirements for zcache (especially for > > > > cleancache pages) and perhaps also for ramster. > > > > > > I don't know requirements for cleancache pages but compaction is > > > general as you know well so I expect you can get a benefit from it > > > if you are concern on memory efficiency but not sure it's valuable > > > to compact cleancache pages for getting more slot in RAM. > > > Sometime, just discarding would be much better, IMHO. > > > > Zcache has page reclaim. Zswap has zpage reclaim. I am > > concerned that these continue to work in the presence of > > compaction. With no reclaim at all, zram is a simpler use > > case but if you implement compaction in a way that can't be > > used by either zcache or zswap, then zsmalloc is essentially > > forking. > > Don't go too far. If it's really problem for zswap and zcache, > maybe, we could add it optionally. Good, I think it should be possible to do it optionally too. > > > > In https://lkml.org/lkml/2013/3/27/501 I suggested it > > > > would be good to work together on a common design, but > > > > you didn't reply. Are you thinking that zsmalloc > > > > > > I saw the thread but explicit agreement is really matter? > > > I believe everybody want it although they didn't reply. :) > > > > > > You can make the design/post it or prototyping/post it. > > > If there are some conflit with something in my brain, > > > I will be happy to feedback. :) > > > > > > Anyway, I think my above statement "COMPACTION" would be enough to > > > express my current thought to avoid duplicated work and you can catch up. > > > > > > I will get around to it after LSF/MM. > > > > > > > improvements should focus only on zram, in which case > > > > > > Just focusing zsmalloc. > > > > Right. Again, I am asking if you are changing zsmalloc in > > a way that helps zram but hurts zswap and makes it impossible > > for zcache to ever use the improvements to zsmalloc. > > As I said, I'm biased to memory efficiency rather than performace. > Of course, severe performance drop is disaster but small drop will > be acceptable for memory-efficiency concerning systems. > > > > If so, that's fine, but please make it clear that is your goal. > > Simple, help memory hungry system. :) One major difference I think is that you are focused on systems where processes often get destroyed by OOMs (e.g. Android-like), where I am focused on server systems where everything possible must be done to avoid killed processes. So IMHO writeback and better integration with the MM system are a requirement. I think that's a key difference between zram and zcache that is driving different design decisions. Dan -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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