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.
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.
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 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
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