Date: Fri, 27 Oct 2006 20:51:04 -0700 From: Julian Elischer <julian@elischer.org> To: "Alexandre \"Sunny\" Kovalenko" <Alex.Kovalenko@verizon.net> Cc: Daniel Eischen <deischen@freebsd.org>, Paul Allen <nospam@ugcs.caltech.edu>, current@freebsd.org Subject: Re: Comments on the KSE option Message-ID: <4542D3A8.1040500@elischer.org> In-Reply-To: <1161999387.872.29.camel@RabbitsDen.RabbitsLawn.verizon.net> References: <45425D92.8060205@elischer.org> <20061027201838.GH30707@riyal.ugcs.caltech.edu> <Pine.GSO.4.64.0610271634160.7105@sea.ntplx.net> <1161998104.872.18.camel@RabbitsDen.RabbitsLawn.verizon.net> <4542B171.8050601@elischer.org> <1161999387.872.29.camel@RabbitsDen.RabbitsLawn.verizon.net>
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Alexandre "Sunny" Kovalenko wrote: > On Fri, 2006-10-27 at 18:25 -0700, Julian Elischer wrote: >> Alexandre "Sunny" Kovalenko wrote: >>> On Fri, 2006-10-27 at 16:41 -0400, Daniel Eischen wrote: >>>> On Fri, 27 Oct 2006, Paul Allen wrote: >>>> >>>>>> From Julian Elischer <julian@elischer.org>, Fri, Oct 27, 2006 at 12:27:14PM -0700: >>>>>> The aim of the fair scheduling code is to ensure that if you, as a user, >>>>>> make a process that starts 1000 threads, and I as a user, make an >>>>>> unthreaded process, then I can still get to the CPU at somewhat similar >>>>>> rates to you. A naive scheduler would give you 1000 cpu slots and me 1. >>>>> Ah. Let me be one of the first to take a crack at attacking this idea as >>>>> a mistake. >>>> No, it is POSIX. You, the application, can write a program with >>>> system scope or process scope threads and get whatever you behavior >>>> you want, within rlimits of course. >>>> >>>> If you want unfair scheduling, then create your threads with >>>> system scope contention, otherwise use process scope. The >>>> kernel should be designed to allow both, and have adjustable >>>> limits in place for (at least) system scope threads. >>>> >>>> Noone is saying that you can't have as many system scope threads >>>> as you want (and as allowed by limits), just that you must also >>>> be able to have process scope threads (with probably higher limits >>>> or possibly no limits). >>>> >>> I might be missing something here, but OP was separating M:N (which is >>> what you are referring to above), and "fairness" (not giving process >>> with 1000 *system scope* threads 1000 CPU scheduling slots). As far as I >>> know the first one is POSIX and the second one is not. >>> >>> FWIW: as an application programmer who spent considerable amount of time >>> lately trying to make heavily multithreaded application run most >>> efficiently on 32-way machine, I would rather not have to deal with >>> "fairness" -- M:N is bad enough. >>> >> >> no, fairness is making sure that 1000 process scope threads >> do not negatively impact other processes. >> 1000 system scope threads are controlled by your ulimit settings >> (Each one counts as a process.) >> >> > I apologize for misinterpreting your words. But then, if I have M:N set > to 10:1, I would expect application with 1000 process scope threads to > have as many CPU slots as 100 processes, or, if I have 10 system scope > threads and 990 process scope threads, I would expect application to > have as many CPU slots as 109 processes. Is this what you refer to as > "fairness"? > M:N is not a ratio, but rather the notation to say that M user threads are enacted using N kernel schedulable entities (kernel threads). usually N is limited to something like NCPU kernel schedulable entities running at a time. (not including sleeping threads waiting for IO) (NCPU is the number of CPUs). so in fact M:N is usually M user threads over over some number like 4 or 8 kernel threads (depending on #cpus) plus the number of threads waiting for IO. Julian
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