Jesper Juhl wrote:
Hi everyone,
This is a continuation of the issue I initially reported in the "make
-j with j <= 4 seems to only load a single CPU core" thread
(http://lkml.org/lkml/2006/2/21/231).
I've now got some more data, so hopefully someone can help me get a
handle on this thing.
In a nutshell the problem is that I need to run "make -j N" where N is
= 5 in order to put maximum load on both cores of my Athlon 64 X2
4400+ when building kernels and with N < 5 the build apears to be
pretty much done serially and not at all parallel. This baffles me to
be honest.
The expected behaviour is that running with "make -j 2" on an
otherwise idle machine would schedule a CC to run on each core most of
the time, and with "make -j 3" & "make -j 4" there should definately
be something executing on both cores all the time.
What I see is that unless I bump it up to -j 5 or greater only one
core seems to be put to work and the other one mostly just sits around
spinning its wheels doing nothing.
Out of concern that this problem may be caused by the smpnice patches
(which modify load balancing) I've done some testing on my HT
workstation (with 2.6.16-rc4 installed). The short summary of the
results of my tests is that the smpnice patches do not appear to be at
fault and that the problem lies in the kernel build mechanism for recent
kernels.
More detail:
Test 1: build linux-2.6.14 with "make -j 2" and observe the results
using top with the "latest CPU ran on" column enabled. Result: both
channels are being used and the build appears to be running in parallel.
Test 2: do the same test with a 2.6.16-rc5 kernel source. Result: both
channels are not being used and the build appears to be running serially.
Hence my conclusion that the problem is caused by recent (since 2.6.14)
changes to the build mechanism and that it is not a scheduler or load
balancing problem.
The good news from this is that a binary search trying to find when the
problem doesn't need to reboot with the kernels each time but just
evaluate the build process.
Peter
--
Peter Williams [email protected]
"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
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