Date: Thu, 13 Jan 2005 21:38:08 +0100 From: Andrea Venturoli <ml.diespammer@netfence.it> To: freebsd-questions@freebsd.org Subject: Re: Hyperthreading hurts 5.3? Message-ID: <41E6DC30.3040501@netfence.it> In-Reply-To: <1242214203.20050113205210@wanadoo.fr> References: <200501121049.j0CAnJQe028309@mp.cs.niu.edu> <828997113.20050112184556@wanadoo.fr> <41E58E53.7060606@netfence.it> <786252184.20050113014354@wanadoo.fr> <41E65688.4010700@netfence.it> <1242214203.20050113205210@wanadoo.fr>
next in thread | previous in thread | raw e-mail | index | archive | help
Anthony Atkielski wrote: > Andrea Venturoli writes: > > AV> Not exactly the same algorithm and on different set of data. > > But similar machine instructions, perhaps? Yes, both numerical computations. Basically one thread would model geometry and the other would mesh it. Frequent stall would arise, as the two process would only by chance require the same time, even so the two CPUs were always at full load (!?!?!?). I also tried different combinations, e.g. three modelling threads and one mesher with, again, equal timings. BTW, it's worth to mention, I *have* to use a compiler that knows nothing about SSE or the like, so all is done with FPU instructions as in the old 387s... > Just the contention for the FPU alone might have had the effect of > single-threading the workload. I've come to the same conclusion. Still I can't put this together with 100% load on both processors. If, as someone said, there is only one FPU, *how* are these figures coming out??? I would have expected something like 50%-50% (instead of 100%-0% of the single threaded version). *If* there is only one FPU, I'd expect both virtual processors being frequently idle waiting for each other. > That plus the SMP overhead might give > you a zero or negative gain with HT. I tried a multithreaded version on a UP machine (nonsense, I know): the locking overhead is there, but very minimal: a process which takes 16 minutes will require, maybe, 3 seconds more. bye av.
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?41E6DC30.3040501>