Re: [RFC v7 00/11] Support vrange for anonymous page

From: John Stultz
Date: Wed Mar 20 2013 - 21:29:53 EST

This is a multi-part message in MIME format.On 03/12/2013 12:38 AM, Minchan Kim wrote:
First of all, let's define the term.
From now on, I'd like to call it as vrange(a.k.a volatile range)
for anonymous page. If you have a better name in mind, please suggest.

This version is still *RFC* because it's just quick prototype so
it doesn't support THP/HugeTLB/KSM and even couldn't build on !x86.
Before further sorting out issues, I'd like to post current direction
and discuss it. Of course, I'd like to extend this discussion in
comming LSF/MM.

In this version, I changed lots of thing, expecially removed vma-based
approach because it needs write-side lock for mmap_sem, which will drop
performance in mutli-threaded big SMP system, KOSAKI pointed out.
And vma-based approach is hard to meet requirement of new system call by
John Stultz's suggested semantic for consistent purged handling.

I tested this patchset with modified jemalloc allocator which was
leaded by Jason Evans(jemalloc author) who was interest in this feature
and was happy to port his allocator to use new system call.
Super Thanks Jason!

The benchmark for test is ebizzy. It have been used for testing the
allocator performance so it's good for me. Again, thanks for recommending
the benchmark, Jason.

The result is good on my machine (12 CPU, 1.2GHz, DRAM 2G)

ebizzy -S 20

jemalloc-vanilla: 52389 records/sec
jemalloc-vrange: 203414 records/sec

ebizzy -S 20 with background memory pressure

jemalloc-vanilla: 40746 records/sec
jemalloc-vrange: 174910 records/sec

And it's much improved on KVM virtual machine.

This patchset is based on v3.9-rc2

- What's the sys_vrange(addr, length, mode, behavior)?

It's a hint that user deliver to kernel so kernel can *discard*
pages in a range anytime. mode is one of VRANGE_VOLATILE and
kernel coudn't discard any pages any more while VRANGE_VOLATILE
is memory unpin opeartion so kernel can discard pages in vrange
anytime. At a moment, behavior is one of VRANGE_FULL and VRANGE
PARTIAL. VRANGE_FULL tell kernel that once kernel decide to
discard page in a vrange, please, discard all of pages in a
vrange selected by victim vrange. VRANGE_PARTIAL tell kernel
that please discard of some pages in a vrange. But now I didn't
implemented VRANGE_PARTIAL handling yet.

So I'm very excited to see this new revision! Moving away from the VMA based approach I think is really necessary, since managing the volatile ranges on a per-mm basis really isn't going to work when we want shared volatile ranges between processes (such as the shmem/tmpfs case Android uses).

Just a few questions and observations from my initial playing around with the patch:

1) So, I'm not sure I understand the benefit of VRANGE_PARTIAL. Why would VRANGE_PARTIAL be useful?

2) I've got a trivial test program that I've used previously with ashmem & my earlier file based efforts that allocates 26megs of page aligned memory, and marks every other meg as volatile. Then it forks and the child generates a ton of memory pressure, causing pages to be purged (and the child killed by the OOM killer). Initially I didn't see my test purging any pages with your patches. The problem of course was the child's COW pages were not also marked volatile, so they could not be purged. Once I over-wrote the data in the child, breaking the COW links, the data in the parent was purged under pressure. This is good, because it makes sure we don't purge cow pages if the volatility state isn't consistent, but it also brings up a few questions:

- Should volatility be inherited on fork? If volatility is not inherited on fork(), that could cause some strange behavior if the data was purged prior to the fork, and also its not clear what the behavior of the child should be with regards to data that was volatile at fork time. However, we also don't want strange behavior on exec if overwritten volatile pages were unexpectedly purged.

- At this moment, maybe not having thought it through enough, I'm wondering if it makes sense to have volatility inherited on fork, but cleared on exec? What are your thoughts here? Its been awhile, so I'm not sure if that's consistent with my earlier comments on the topic.

3) Oddly, in my test case, once I changed the child to over-write the volatile range and break the COW pages, the OOM killer more frequently seems to favor killing the parent process, instead of the memory hogging child process. I need to spend some more time looking at this, and I know the OOM killer may go for the parent process sometimes, but it definitely happens more frequently then when the COW pages are not broken and no data is purged. Again, I need to dig in more here.

4) One of the harder aspects I'm trying to get my head around is how your patches seem to use both the page list shrinkers (discard_vpage) to purge ranges when particular pages selected, and a zone shrinker (discard_vrange_pages) which manages its own lru of vranges. I get that this is one way to handle purging anonymous pages when we are on a swapless system, but the dual purging systems definitely make the code harder to follow. Would something like my earlier attempts at changing vmscan to shrink anonymous pages be simpler? Or is that just not going to fly w/ the mm folks?

I'll continue working with the patches and try to get tmpfs support added here soon.

Also, attached is a simple cleanup patch that you might want to fold in.