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Old 2019-09-04, 19:47   #1
Jan 2015

11×23 Posts
Default How much of a problem is jitter?

Still reading, and a lot of this is written for Xeons, but it sounds like every time there's a C or P state change to either boost performance or save power there's a 15 microsecond CPU delay, which to my layman's ears sounds like kind of a lot, especially in the CPU realm when other things like RAM have latencies of 70ns and infiniband has 300ns.

Unless I'm wrong about things I already know any CPU will never hit advertised turbo boost speeds at 100% phyCPU utilization.

I believe there's also things that save power at the cost of performance on the RAM and IO side of things, kind of wondering what the entire big picture is.

I could test out each of these workload profiles and use our 100M benchmark exponent 332220523 to see how it impacts things....what do you all think the number of iterations would need to be to get an accurate representation? test box is a single socket DL360, intel 6150 gold, 384GB of ddr4 2133
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Old 2019-09-12, 01:56   #2
NookieN's Avatar
Aug 2002

2·29 Posts

It depends on your perspective maybe. At 2.4GHz, 15us is, what, 36,000 ticks? If you re-lock to 2.5GHz you should recover the lost time if you can stay at that frequency for more than 360us. Presumably the Pcode knows that and factors it in to decide if it will burn through its thermal headroom faster or slower than that.

As far as hitting advertised turbo, the specs list different max turbo frequencies for number of active cores. So they may advertise 4GHz as the headline number for 1 core and 2.8GHz for 16 cores, even if the nominal speed of the chip is 2.4GHz. It should sustain that turbo frequency for the number of active cores as long as the thermals allow. AVX and AVX512 also affect the max turbo the Pcode will allow, regardless of cooling.
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