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SubjectRe: [PATCH 0/8][for -mm] mem_notify v6
I just noticed this patchset, kosaki-san.  It looks quite interesting;
my apologies for not commenting earlier.

I see mention somewhere that mem_notify is of particular interest to
embedded systems.

I have what seems, intuitively, a similar problem at the opposite
end of the world, on big-honkin NUMA boxes (hundreds or thousands of
CPUs, terabytes of main memory.) The problem there is often best
resolved if we can kill the offending task, rather than shrink its
memory footprint. The situation is that several compute intensive
multi-threaded jobs are running, each in their own dedicated cpuset.

If one of these jobs tries to use more memory than is available in
its cpuset, then

(1) we quickly loose any hope of that job continuing at the excellent
performance needed of it, and

(2) we rapidly get increased risk of that job starting to swap and
unintentionally impact shared resources (kernel locks, disk
channels, disk heads).

So we like to identify such jobs as soon as they begin to swap,
and kill them very very quickly (before the direct reclaim code
in mm/vmscan.c can push more than a few pages to the swap device.)

For a much earlier, unsuccessful, attempt to accomplish this, see:

[Patch] cpusets policy kill no swap
http://lkml.org/lkml/2005/3/19/148

Now, it may well be that we are too far apart to share any part of
a solution; one seldom uses the same technology to build a Tour de
France bicycle as one uses to build a Lockheed C-5A Galaxy heavy
cargo transport.

One clear difference is the policy of what action we desire to take
when under memory pressure: do we invite user space to free memory so
as to avoid the wrath of the oom killer, or do we go to the opposite
extreme, seeking a nearly instantant killing, faster than the oom
killer can even begin its search for a victim.

Another clear difference is the use of cpusets, which are a major and
vital part of administering the big NUMA boxes, and I presume are not
even compiled into embedded kernels (correct?). This difference maybe
unbridgeable ... these big NUMA systems require per-cpuset mechanisms,
whereas embedded may require builds without cpusets.

However ... there might be some useful cross pollination of ideas.

I see in the latest posts to your mem_notify patchset v6, responding
to comments by Andrew and Andi on Feb 12 and 13, that you decided to
think more about the design of this, so perhaps this is a good time
for some random ideas from myself, even though I'm clearly coming from
a quite different problem space in some ways.

1) You have a little bit of code in the kernel to throttle the
thundering herd problem. Perhaps this could be moved to user space
... one user daemon that is always notified of such memory pressure
alarms, and in turn notifies interested applications. This might
avoid the need to add poll_wait_exclusive() to the kernel. And it
moves any fussy details of how to tame the thundering herd out of
the kernel.

2) Another possible mechanism for communicating events from
the kernel to user space is inotify. For example, I added
the line:

fsnotify_modify(dentry); # dentry is current tasks cpuset

at an interesting spot in vmscan.c, and using inotify-tools
<inotify-tools.sourceforge.net/> could easily watch all cpusets
for these events from one user space daemon.

At this point, I have no idea whether this odd use of inotify
is better or worse than what your patchset has. However using
inotify did require less new kernel code, and with such user space
mechanisms as inotify-tools already well developed, it made the
problem I had, of watching an entire hierarcy of special files
(beneath /dev/cpuset) very easy to implement. At least inotify
also presents events on a file descriptor that can be consumed
using a poll() loop.

3) Perhaps, instead of sending simple events, one could update
a meter of the rate of recent such events, such as the per-cpuset
'memory_pressure' mechanism does. This might lead to addressing
Andrew Morton's comment:

If this feature is useful then I'd expect that some
applications would want notification at different times, or at
different levels of VM distress. So this semi-randomly-chosen
notification point just won't be strong enough in real-world
use.
4) A place that I found well suited for my purposes (watching for
swapping from direct reclaim) was just before the lines in the
pageout() routine in mm/vmscan.c:

if (clear_page_dirty_for_io(page)) {
...
res = mapping->a_ops->writepage(page, &wbc);
It seemed that testing "PageAnon(page)" here allowed me to easily
distinguish between dirty pages going back to the file system, and
pages going to swap (this detail is from work on a 2.6.16 kernel;
things might have changed.)

One possible advantage of the above hook in the direct reclaim
code path in vmscan.c is that pressure in one cpuset did not cause
any false alarms in other cpusets. However even this hook does
not take into account the constraints of mm/mempolicy (the NUMA
memory policy that Andi mentioned) nor of cgroup memory controllers.

5) I'd be keen to find an agreeable way that you could have the
system-wide, no cpuset, mechanism you need, while at the same
time, I have a cpuset interface that is similar and depends on the
same set of hooks. This might involve a single set of hooks in
the key places in the memory and swapping code, that (1) updated
the system wide state you need, and (2) if cpusets were present,
updated similar state for the tasks current cpuset. The user
visible API would present both the system-wide connector you need
(the special file or whatever) and if cpusets are present, similar
per-cpuset connectors.

Anyhow ... just some thoughts. Perhaps one of them will be useful.

--
I won't rest till it's the best ...
Programmer, Linux Scalability
Paul Jackson <pj@sgi.com> 1.940.382.4214


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