[Devel] Re: controlling mmap()'d vs read/write() pages

[Eric W. Biederman]

Dave Hansen <hansendc at us.ibm.com> writes:

>
> So, I think we have a difference of opinion.  I think it's _all_ about
> memory pressure, and you think it is _not_ about accounting for memory
> pressure. :)  Perhaps we mean different things, but we appear to
> disagree greatly on the surface.

I think it is about preventing a badly behaved container from having a
significant effect on the rest of the system, and in particular other
containers on the system.

See below.  I think to reach agreement we should start by discussing
the algorithm that we see being used to keep the system function well
and the theory behind that algorithm.  Simply limiting memory is not
enough to understand why it works....

> Can we agree that there must be _some_ way to control the amounts of
> unmapped page cache?  Whether that's related somehow to the same way we
> control RSS or done somehow at the I/O level, there must be some way to
> control it.  Agree?

At lot depends on what we measure and what we try and control.
Currently what we have been measuring are amounts of RAM, and thus
what we are trying to control is the amount of RAM.  If we want to
control memory pressure we need a definition and a way to measure it.
I think there may be potential if we did that but we would still need
a memory limit to keep things like mlock in check.


So starting with a some definitions and theory.
RSS is short for resident set size.  The resident set being how many
of pages are current in memory and not on disk and used by the
application.  This includes the memory in page tables, but can
reasonably be extended to include any memory a process can be shown to
be using.

In theory there is some minimal RSS that you can give an application
at which it will get productive work done.  Below the minimal RSS
the application will spend the majority of real time waiting for
pages to come in from disk, so it can execute the next instruction.
The ultimate worst case here is a read instruction appearing on one
page and it's datum on another.  You have to have both pages in memory
at the same time for the read to complete.  If you set the RSS hard
limit to one page the problem will be continually restarting either
because the page it is on is not in memory or the page it is reading
from is not in memory.

What we want to accomplish is to have a system that runs multiple
containers without problems.  As a general memory management policy
we can accomplish this by ensuring each container has at least
it's minimal RSS quota of pages.  By watching the paging activity
of a container it is possible to detect when that container has
to few pages and is spend all of it's time I/O bound, and thus
has slipped below it's minimal RSS.

As such it is possible for the memory management system if we have
container RSS accounting to dynamically figure out how much memory
each container needs and to keep everyone above their minimal RSS
most of the time when that is possible.  Basically to do this the
memory manage code would need to keep dynamic RSS limits, and
adjust them based upon need.

There is still the case when not all containers can have their
minimal RSS, there is simply not enough memory in the system.

That is where having a hard settable RSS limit comes in.  With this
we communicate to the application and the users beyond which point we
consider their application to be abusing the system.


There is a lot of history with RSS limits showing their limitations
and how they work.  It is fundamentally a dynamic policy instead of
a static set of guarantees which allows for applications with a
diverse set of memory requirements to work in harmony.


One of the very neat things about a hard RSS limit is that if there
are extra resources on the system you can improve overall system
performance by cache pages in the page cache instead writing them
to disk.

> http://linux-mm.org/SoftwareZones

I will try and take a look in a bit.


Eric
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