Comment by rwmj

Comment by rwmj 13 hours ago

Let me test that now. Note I only have 1 Intel machine so any results are very specific to this laptop.

  -j           time (mean ± σ)
  12 (#P+#E)   130.889 s ±  4.072 s
  13 (..+1)    135.049 s ±  2.270 s
   4 (#P)      179.845 s ±  1.783 s
   8 (#E)      141.669 s ±  3.441 s

Machine: 13th Gen Intel(R) Core(TM) i7-1365U; 2 x P-cores (4 threads), 8 x E-cores

wtallis 11 hours ago

Your processor has two P cores, and ten cores total, not twelve. The HyperThreading (SMT) does not make the two P cores into four cores. Your experiment with 4 threads will most likely result in using both P cores and two E cores, as no sane OS would double up threads on the P cores before the E cores were full with one thread each.

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rwmj 11 hours ago

The hyperthreading should cover up memory latency, since the workload (compiling qemu) might not fit into L3 cache. Although I take your point that it doesn't magically create two core-equivalents.

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- gonzo 8 hours ago
  
  “Hyperthreading” is a write pipe hack.
  If the core stalls on a write then the other thread gets run.
  
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jeffbee 11 hours ago

I am sure rwmj was smart enough to use `taskset` to make this experiment meaningful.

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- rwmj 11 hours ago
  
  Hehe, if only :-( However I do want to know what's best with the default Linux scheduler and just using 'make' rather than more complicated commands.
  
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