Date: Sat, 26 Aug 2017 10:28:50 +1000 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Don Lewis <truckman@freebsd.org> Cc: avg@freebsd.org, freebsd-arch@freebsd.org Subject: Re: ULE steal_idle questions Message-ID: <20170826094725.G1648@besplex.bde.org> In-Reply-To: <201708251824.v7PIOA6q048321@gw.catspoiler.org> References: <201708251824.v7PIOA6q048321@gw.catspoiler.org>
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On Fri, 25 Aug 2017, Don Lewis wrote: > ... > Something else that I did not expect is the how frequently threads are > stolen from the other SMT thread on the same core, even though I > increased steal_thresh from 2 to 3 to account for the off-by-one > problem. This is true even right after the system has booted and no > significant load has been applied. My best guess is that because of > affinity, both the parent and child processes run on the same CPU after > fork(), and if a number of processes are forked() in quick succession, > the run queue of that CPU can get really long. Forcing a thread > migration in exec() might be a good solution. Since you are trying a lot of combinations, maybe you can tell us which ones work best. SCHED_4BSD works better for me on an old 2-core system. SCHED_ULE works better on a not-so old 4x2 core (Haswell) system, but I don't like it due to its complexity. It makes differences of at most +-2% except when mistuned it can give -5% for real time (but better for CPU and presumably power). For SCHED_4BSD, I wrote fancy tuning for fork/exec and sometimes get everything to like up for a 3% improvement (803 seconds instead of 823 on the old system, with -current much slower at 840+ and old versions of ULE before steal_idle taking 890+). This is very resource (mainly cache associativity?) dependent and my tuning makes little difference on the newer system. SCHED_ULE still has bugfeatures which tend to help large builds by reducing context switching, e.g., by bogusly clamping all CPU-bound threads to nearly maximal priority. Bruce
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