Re: [RFC v2] Support volatile range for anon vma

[Minchan Kim]

Hi John,

Sorry for the long delay.
At last, I get a chance to look at this, again.

On Wed, Nov 21, 2012 at 04:36:42PM -0800, John Stultz wrote:
> On 10/29/2012 06:29 PM, Minchan Kim wrote:
> >This patch introudces new madvise behavior MADV_VOLATILE and
> >MADV_NOVOLATILE for anonymous pages. It's different with
> >John Stultz's version which considers only tmpfs while this patch
> >considers only anonymous pages so this cannot cover John's one.
> >If below idea is proved as reasonable, I hope we can unify both
> >concepts by madvise/fadvise.
> >
> >Rationale is following as.
> >Many allocators call munmap(2) when user call free(3) if ptr is
> >in mmaped area. But munmap isn't cheap because it have to clean up
> >all pte entries and unlinking a vma so overhead would be increased
> >linearly by mmaped area's size.
> >
> >Volatile conecept of Robert Love could be very useful for reducing
> >free(3) overhead. Allocators can do madvise(MADV_VOLATILE) instead of
> >munmap(2)(Of course, they need to manage volatile mmaped area to
> >reduce shortage of address space and sometime ends up unmaping them).
> >The madvise(MADV_VOLATILE|NOVOLATILE) is very cheap opeartion because
> >
> >1) it just marks the flag in VMA and
> >2) if memory pressure happens, VM can discard pages of volatile VMA
> >    instead of swapping out when volatile pages is selected as victim
> >    by normal VM aging policy.
> >3) freed mmaped area doesn't include any meaningful data so there
> >    is no point to swap them out.
> >
> >Allocator should call madvise(MADV_NOVOLATILE) before reusing for
> >allocating that area to user. Otherwise, accessing of volatile range
> >will meet SIGBUS error.
> >
> >The downside is that we have to age anon lru list although we don't
> >have swap because I don't want to discard volatile pages by top priority
> >when memory pressure happens as volatile in this patch means "We don't
> >need to swap out because user can handle the situation which data are
> >disappear suddenly", NOT "They are useless so hurry up to reclaim them".
> >So I want to apply same aging rule of nomal pages to them.
> >
> >Anon background aging of non-swap system would be a trade-off for
> >getting good feature. Even, we had done it two years ago until merge
> >[1] and I believe free(3) performance gain will beat loss of anon lru
> >aging's overead once all of allocator start to use madvise.
> >(This patch doesn't include background aging in case of non-swap system
> >  but it's trivial if we decide)
> 
> Hey Minchan!
>     So I've been looking at your patch for a bit, and I'm still
> trying to fully grok it and the rmap code. Overall this approach
> looks pretty interesting,  and while your patch description focused
> on malloc/free behavior, I suspect your patch would satisfy what the
> mozilla folks are looking for, and while its not quite sufficient
> yet for Android, the interface semantics are very close to what I've
> been wanting (my test cases were easily mapped over).
> 
> The two major issues for me are:
> 1) As you noted, this approach currently doesn't work on non-swap
> systems, as we don't try to shrink the anonymous page lrus. This is
> a big problem, as it makes it unusable for most all Android systems.
> You suggest we may want to change aging the anonymous lru, and I had
> a patch earlier that tried to change some of the anonymous lru aging
> rules for volatile pages, but its not quite right for what you have
> here. So I'd be interested in hearing how you think the anonymous
> lru aging should happen with swapoff.

As I mentioned, anon LRU aging should happen for getting the benefit
of VOLATILE. I expect user-space allocator(ex, glibc) or VM heap management
will start it so the gain would be higher than lose.
Otherwise, we should choose and move volatile pages into anonther LRU
when madvise calls. It would be a big overhead as you already have measured.
So the design is same with rmap which move the overhead from frequent place
(ex, fork, pgfault) to to rare plcae(ie, reclaim)
I will accept if there is another good idea which is able to minimise
madvise's overhead.

> 
> 2) Being able to use this with tmpfs files. I'm currently trying to
> better understand the rmap code, looking to see if there's a way to
> have try_to_unmap_file() work similarly to try_to_unmap_anon(), to
> allow allow users to madvise() on mmapped tmpfs files. This would
> provide a very similar interface as to what I've been proposing with
> fadvise/fallocate, but just using process virtual addresses instead
> of (fd, offset) pairs.   The benefit with (fd,offset) pairs for
> Android is that its easier to manage shared volatile ranges between
> two processes that are sharing data via an mmapped tmpfs file
> (although this actual use case may be fairly rare).  I believe we
> should still be able to rework the ashmem internals to use madvise
> (which would provide legacy support for existing android apps), so
> then its just a question of if we could then eventually convince
> Android apps to use the madvise interface directly, rather then the
> ashmem unpin ioctl.

I didn't look at ashmem in detail. let's pass the answer to andorid
folks. But apparently, it's very important. If we can't, We might
separate madvise(anon)/fallocate(file).

> 
> The other concern with the madvise on mmapped files approach is that
> there's no easy way I can see to limit it to tmpfs files. I know

First thing I can thing of is PG_swapbacked. I guess It would be set
to only swappable data.

> some have been interested in having fallocate(VOLATILE) interface
> for non-tmpfs files, but I'm not sure I see the benefit there yet. I
> have noted folks mixing the idea of volatile pages being purged
> under memory pressure with the idea of volatile files, which might
> be purged from disk under disk pressure. While I think the second
> idea is interesting, I do think its completely separate from the
> volatile memory concept.
> 
> Anyway, I'd be interested in your thoughts on these two issues.
> Thanks so much for sending out this patch, its given me quite a bit
> to chew on, and I too hope we can merge our different approaches
> together.

Thanks, John!

> 
> thanks
> -john
> 
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-- 
Kind regards,
Minchan Kim
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