Re: [PATCH 0/5][RFC] Fallocate Volatile Ranges v6

John Stultz
2012-08-09T18:46:25Z

On 08/09/2012 02:28 AM, Michel Lespinasse wrote:
> Hi John,
>
> On Fri, Jul 27, 2012 at 8:57 PM, John Stultz <john.stultz@linaro.org> wrote:
>> So after not getting too much positive feedback on my last
>> attempt at trying to use a non-shrinker method for managing
>> & purging volatile ranges, I decided I'd go ahead and try
>> to implement something along Minchan's ERECLAIM LRU list
>> idea.
> Agree that there hasn't been much feedback from MM folks yet - sorry
> about that :/
>
> I think one issue might be that most people don't have a good
> background on how the feature is intended to be used, and it is very
> difficult to comment meaningfully without that.
>
> As for myself, I have been wondering:
>
> - Why the feature needs to be on a per-range basis, rather than
> per-file. Is this simply to make it easier to transition the android
> use case from whatever they are doing right now, or is it that the
> object boundaries within a file can't be known in advance, and thus
> one wouldn't know how to split objects accross different files ? Or
> could it be that some of the objects would be small (less than a page)
> so space use would be inefficient if they were placed in different
> files ? Or just that there would be too many files for efficient
> management ?
For me, keeping the feature per-range instead of something like per-file
is in order to be able to support Android's existing use case.

As to why Android uses per-range instead of per file, Arve or someone
from the Android team could probably better answer, but I can
theorize. In discussions with the Android guys, they've mentioned
ashmem's primary goal was to basically provide atomically unlinked tmpfs
fds for sharing memory, and memory unpinning was a follow on feature.
So I suspect that ranges fit better into their existing model of using a
mmapped fd to share data between two processes. Instead of creating a
new tmpfs file and sharing the fd for each object, they're able to
create a one shared mapping, which can be effectively resized w/
unpinning, for multiple objects.

Another use case where ranges would be beneficial might be for where
there's a large single object that might not all be in use at one time.
For example, a very large web page, where you have a limited view onto
it. Having the rest of the page rendered so you can quickly scroll
without re-rendering would be nice, but under memory pressure it could
effectively throw out the non-visible portions.

Further, one additional reason I have for not having the volatile
attribute be per-file, is that I have my eye on allowing volatile ranges
to be set on anonymous heap memory via something like madvise() in the
future, which would be an easier api to use if you're not sharing data
via mmapped tmpfs files, but wouldn't be possible if this was a file
attribute rather then a range of pages.

> - What are the desired semantics for the volatile objects. Can the
> objects be accessed while they are marked as volatile, or do they have
> to get unmarked first ?
So accessing a volatile page before marking it non-volatile can produce
undefined behavior. You could get the data that was there, or you could
get empty pages. The expectation is that pages are unmarked before
being accessed, so one can know if the data was lost or not. I'm open
to other suggestions here, if folks think we should SIGSEGV on accesses
to volatile pages. However, I don't know how setting that up and tearing
it down on each mark_volatile/unmark_volatile might affect performance.

> Is it really the case that we always want to
> reclaim from volatile objects first, before any other kind of caches
> we might have ? This sounds like a very strong hint, and I think I
> would be more comfortable with something more subtle if that's
> possible.
So the current Android ashmem implementation uses a shrinker, which
isn't necessarily called before any other caches are freed. So, I don't
think its a strong hint, but it just seems somewhat intuitive to me that
we should free effectively "user-donated" pages before freeing other
system caches. But that's not something the interface necessarily
defines or requires.

> Also, if we have several volatile objects to reclaim from,
> is it desirable to reclaim from the one that's been marked volatile
> the longest or does it make no difference ?
While I don't think its strictly necessary, I do think LRU order purging
is important from the least-surprise angle. Since ranges marked
volatile should not be touched until they are marked non-volatile, it
follows normal expectations that recently touched data is likely to be
faster then data that has not been accessed for some time. Reasonable
exceptions would be situations like NUMA systems where pressure on one
node forces purging volatile pages in a non-global-lru order. So
probably not critical, but I think useful to try to preserve.

> When an object is marked
> volatile, would it be sufficient to ensure it gets placed on the
> inactive list (maybe with the referenced bit cleared) and let the
> normal reclaim algorithm get to it, or is that an insufficiently
> strong hint somehow ?

The problem is that on machines without swap, there wouldn't be any
reclaim for inactive anonymous pages.

An earlier iteration of this implementation (v4 I think?) used writepage
to decide if to purge or writeout a page, and made it so in non-swap
cases the anonymous inactive list was in effect the "volatile" list.
However, folks didn't seem to like this approach.

> Basically, having some background information of how android would be
> using the feature would help us better understand the design decision
> here, I think.
Hopefully the details above help, and I'll try to get some more concrete
examples from the Android code base.

thanks
-john