Date: Sat, 26 Aug 2017 12:18:14 -0600 From: Ian Lepore <ian@freebsd.org> To: "Rodney W. Grimes" <freebsd-rwg@pdx.rh.CN85.dnsmgr.net>, 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: <1503771494.56799.49.camel@freebsd.org> In-Reply-To: <201708261812.v7QIC2eJ074443@pdx.rh.CN85.dnsmgr.net> References: <201708261812.v7QIC2eJ074443@pdx.rh.CN85.dnsmgr.net>
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On Sat, 2017-08-26 at 11:12 -0700, Rodney W. Grimes wrote: > > > > 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? > I would second that sentiment... as time goes on, heavily loaded systems seem to become less and less interactive-friendly. Also, running the heavy-load jobs such as builds with nice, even -n 20, doesn't seem to make any noticible difference in terms of making un- nice'd processes more responsive (not sure there's any relationship in the underlying causes of that, though). -- Ian
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