Re: [PATCH 0/8] RSS controller based on process containers (v3.1)

Herbert Poetzl
2007-06-08T15:37:33Z

On Fri, Jun 08, 2007 at 04:39:28PM +0400, Pavel Emelianov wrote:
> Herbert Poetzl wrote:
> > On Mon, Jun 04, 2007 at 05:25:25PM +0400, Pavel Emelianov wrote:
> >> Adds RSS accounting and control within a container.
> >>
> >> Changes from v3
> >> - comments across the code
> >> - git-bisect safe split
> >> - lost places to move the page between active/inactive lists
> >>
> >> Ported above Paul's containers V10 with fixes from Balbir.
> >>
> >> RSS container includes the per-container RSS accounting
> >> and reclamation, and out-of-memory killer.
> >>
> >>
> >> Each mapped page has an owning container and is linked into its
> >> LRU lists just like in the global LRU ones. The owner of the page
> >> is the container that touched the page first.
> >
> >> As long as the page stays mapped it holds the container, is accounted
> >> into its usage and lives in its LRU list. When page is unmapped for
> >> the last time it releases the container.
> >
> >> The RSS usage is exactly the number of pages in its booth LRU lists,
> >> i.e. the nu,ber of pages used by this container.
> >
> > so there could be two guests, unified (i.e. sharing
> > most of the files as hardlinks), where the first one
> > holds 80% of the resulting pages, and the second one
> > 20%, and thus shows much lower 'RSS' usage as the
> > other one, although it is running the very same
> > processes and providing identical services?
>
> Herbert!!! Where have you been so long?

I was on vacation in april, and it took almost the
entire may to process the backlog ...

> You must have missed that we've decided not to account pages
> sharing right now, but start with that model. Later we'll make
> sharing accountable.

well, there are two ways not to account sharing:

1) account it to the first user
2) account it to every user

while the first one typically causes quite an
imbalance when applied to shared resources (as
Linux-VServer uses them), the latter one creates
a new, but fair, metric for the usage

to make that clear: we definitely prefer the
latter one over the former, because we do not
want to bring that imbalance to our shared guests
(besides that, the second one doesn't add any
overhead compared to the first one, more than that
it probably simplifies the entire design)

> ( There's something bad with my memory. I have a feeling that I
> have already told that... many times... )

yeah, I have some kind of déjà vu here too ...

> >> When this usage exceeds the limit set some pages are reclaimed
> >> from the owning container. In case no reclamation possible the OOM
> >> killer starts thinning out the container.
> >
> > so the system (physical machine) starts reclaiming
> > and probably swapping even when there is no need
> > to do so?
>
> Good catch! The system will start reclaiming right when the
> container hits the limit to expend its IO bandwidth. Not some
> other's one that hit the global limit due to some bad container
> was allowed to go above it.

well, from the system PoV, a constantly swapping
guest (on an otherwise unused host) is definitely
something you do not really want, not to talk
about a tightly packed host system, where guests
start hogging the I/O with _unnecessary_ swapping

> > e.g. a system with a single guest, limited to 10k
> > pages, with a working set of 15k pages in different
> > apps would continuously swap (trash?) on an otherwise
> > unused (100k+ pages) system?
> >
> >> Thus the container behaves like a standalone machine -
> >> when it runs out of resources, it tries to reclaim some
> >> pages, and if it doesn't succeed, kills some task.
> >
> > is that really what we want?
>
> A kind of ;)

okay, to clarify, we (Linux-VServer) do not want
that behavior ...

> > I think we can do _better_ than a standalone machine
> > and in many cases we really should ...
>
> That's it! You are right - this is our ultimate goal. And we
> plan to get there step by step. And we will appreciate your
> patches fixing BUGS, improving the performance, extending the
> functionality, etc.

well, I would rip out the entire accounting and
add a summation accounting as we do it right now,
no problem with keeping the reclaim mechanisms
though ... but I doubt that this is what you have
in mind?

> > best,
> > Herbert
>
> Thanks for your attention,
> Pavel

just to make it clear, I don't want any limits
in mainline which penalize the first started
guest in a shared guest scenario .. or to rephrase,
such a limit would be useless for our purpose

best,
Herbert

> >> Signed-off-by: Pavel Emelianov <xemul@openvz.org>
> >>
> >> The testing scenario may look like this:
> >>
> >> 1. Prepare the containers
> >> # mkdir -p /containers/rss
> >> # mount -t container none /containers/rss -o rss
> >>
> >> 2. Make the new group and move bash into it
> >> # mkdir /containers/rss/0
> >> # echo $$ > /containers/rss/0/tasks
> >>
> >> Since now we're in the 0 container.
> >> We can alter the RSS limit
> >> # echo -n 6000 > /containers/rss/0/rss_limit
> >>
> >> We can check the usage
> >> # cat /containers/rss/0/rss_usage
> >> 25
> >>
> >> And do other stuff. To check the reclamation to work we need a
> >> simple program that touches many pages of memory, like this:
> >>
> >> #include <stdio.h>
> >> #include <unistd.h>
> >> #include <sys/mman.h>
> >> #include <fcntl.h>
> >>
> >> #ifndef PGSIZE
> >> #define PGSIZE 4096
> >> #endif
> >>
> >> int main(int argc, char **argv)
> >> {
> >> unsigned long pages;
> >> int i;
> >> char *mem;
> >>
> >> if (argc < 2) {
> >> printf("Usage: %s <number_of_pages>\n", argv[0]);
> >> return 1;
> >> }
> >>
> >> pages = strtol(argv[1], &mem, 10);
> >> if (*mem != '\0') {
> >> printf("Bad number %s\n", argv[1]);
> >> return 1;
> >> }
> >>
> >> mem = mmap(NULL, pages * PGSIZE, PROT_READ | PROT_WRITE,
> >> MAP_PRIVATE | MAP_ANON, 0, 0);
> >> if (mem == MAP_FAILED) {
> >> perror("map");
> >> return 2;
> >> }
> >>
> >> for (i = 0; i < pages; i++)
> >> mem[i * PGSIZE] = 0;
> >>
> >> printf("OK\n");
> >> return 0;
> >> }
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> >