Date: Sat, 28 Oct 2006 20:44:55 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Robert Watson <rwatson@freebsd.org> Cc: Julian Elischer <julian@elischer.org>, Paul Allen <nospam@ugcs.caltech.edu>, David Xu <davidxu@freebsd.org>, freebsd-current@freebsd.org Subject: Re: Comments on the KSE option Message-ID: <200610290344.k9T3itAw054920@apollo.backplane.com> References: <45425D92.8060205@elischer.org> <200610281132.21466.davidxu@freebsd.org> <20061028105454.S69980@fledge.watson.org> <20061028194125.GL30707@riyal.ugcs.caltech.edu> <20061028204357.A83519@fledge.watson.org>
next in thread | previous in thread | raw e-mail | index | archive | help
:I think the notion of fairness is orthogonal to M:N threading. M:N is about
:efficiently representing user threading to kernel space, as well as avoiding
:kernel involvement in user context switches when not needed. Fairness is
:about how the kernel allocates time slices to user processes/threads.
:Fairness can be implemented for both 1:1 and M:N, with the primary differences
:being in bookkeeping.
Yes, this is precisely what I mean. Very well said.
What we are talking about here is primarily algorithmic complexity
and physical resource limitations (e.g. like kernel memory). Having
the kernel scheduler only deal with (N) threads, where N is limited by
the number of physical cpus, is a far easier problem for the kernel
to solve in all respects then having the kernel deal with (M*N)
individual threads. I personally see no reason why a program
couldn't have 10,000 threads, or 100,000 threads, or a million threads,
but the kernel is the wrong place to try to manage them if your system
only has N cpus (N=2,4,8,16,32, etc). You have to ask yourself, what
exactly is the kernel accomplishing trying to manage all those threads
for a single application when it only has N cpu contexts to work with
anyhow? The answer is: The kernel should only have to worry about the
N cpu contexts and kernel memory reosurces for those contexts.
----
From the point of view of POSIX threading and a resource limits, people
need to understand two things:
(1) setrlimit was NEVER designed as a system moderation tool. It
was designed to cause runaway programs to fail, period. setrlimit
cannot, in fact, be used as a system moderation tool. Not very well
anyway. setrlimit especially breaks down when you have a huge range
of acceptable values, because higher values tend to muliply out and
you wind up losing the protection that setrlimit was designed to
supply. A good example of this is having a per-process descriptor
limit AND a per-user process limit. X*Y often exceeds the size of
the kernel's global descriptor table. Oops!
(2) Just because the POSIX scheduler implements all sorts of different
scopes and priority schemes says NOTHING AT ALL about how programs
operating under such a scheduler should be apportioned cpu relative
to OTHER PROGRAMS WHICH ARE INDEPENDANTLY RUNNING ON THE SYSTEM. POSIX
is an abstraction (or virtualization out of available resources),
just like everything else. If you try to treat it as a hard requirement
the only result will be a broken system that might happily run everything
else into the ground and stop allowing root ssh logins in order to
accomodate a badly written POSIX program. There are many third party
applications that set POSIX priorities, in particular realtime
priorities, that I'd rather they not actually use. Most of these
programs set these priorities based on the author's attempt to tune
them on a single operating system (e.g. linux) and in a single operating
environment.
All a program can ever really do when requesting POSIX scheduling
resources is compete against itself. It is the system operator, at a
higher level, that must control how those resources compete with
other programs. That should be clear to everyone it is so obvious.
It is a whole lot easier for the kernel to give the system operator
this power if the kernel scheduler does not have to juggle thousands
of threads. It is very easy to write a scheduler for threaded
applications when the most you have to deal with is N threads
(N=ncpus) per application.
--
Now lets consider programs which fork() instead of thread. The argument
that threading is equivalent to forking from a management standpoint
is just plain silly. From a design standpoint, programmers are very
well aware of the resources required to fork(), and consequently
per-fork tasks are generally much, MUCH better understood by system
operators in the management context then per-thread tasks. per-thread
tasks tend to be opaque... you never know how a threaded program might
be written. You just cannot treat the two as equivalent or even close
to equivalent.
-Matt
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200610290344.k9T3itAw054920>