Date: Sat, 26 Aug 2017 11:12:02 -0700 (PDT) From: "Rodney W. Grimes" <freebsd-rwg@pdx.rh.CN85.dnsmgr.net> To: Bruce Evans <brde@optusnet.com.au> Cc: Don Lewis <truckman@freebsd.org>, avg@freebsd.org, freebsd-arch@freebsd.org Subject: Re: ULE steal_idle questions Message-ID: <201708261812.v7QIC2eJ074443@pdx.rh.CN85.dnsmgr.net> In-Reply-To: <20170826094725.G1648@besplex.bde.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. That last bugfeature is probably what makes current systems interactive performance tank rather badly when under heavy loads. Would it be hard to fix? -- Rod Grimes rgrimes@freebsd.org
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