On 6/18/13 12:01 PM, Dirk Jagdmann wrote:
>> How many processor cores does your machine have, and are they
>> multi-threaded?
>>
>> I have a 4-dual-threaded-core machine (and a solid-state disk and a
>> lot of memory), and "-j 8" seems to run the CPU at about 100%. I
>> don't know whether "number of threads" would be a good default in all
>> cases - what you really want is to keep cranking up the number of jobs
>> until things don't get faster, but that's a pain - but, if it is a
>> good default, then
>>
>> sysctl -n hw.logicalcpu
>>
>> would give cores*threads-per-core (tested on Snow Leopard, Lion, and
>> Mountain Lion; the script doesn't work on Leopard, as the versions of
>> at least some of the software won't configure and build, and I gave up
>> trying to make them build).
>>
>> (I originally used -j 4, but then saw something about a multi-threaded
>> version of the IBM System/360 Model 195 that noted that
>> multi-threading can help deal with pipeline bubbles due to unpredicted
>> or mispredicted branches, so I tried -j 8 and found the CPU was busier
>> than with -j 4.)
>
> hmm, there are many points discussed here:
>
> - do we want to make a smart decision on the number of parallel make
> jobs, possibly derived from the number of cores?
>
> My personal opinion is yes. On OsX using the sysctl mentioned by Guy is
> probably a good starting point. Adding my own experience you'll want to
> oversubscribe the number of jobs over the number of cores a little bit,
> so that when jobs are waiting for I/O action another job can use the
> free CPU. With my personal experience calculating something like "number
> of cores"*1.5 seems reasonable, because if you're waiting for the long
> lasting build job on your desktop computer, you want some "juice" left
> over for web browsing, email reading or whatever. (This applies to all
> modern operating system, Windows, Linux, OsX)
>
> - the other question is, which versions of OsX are we targetting for
> developer support going forward. Personally I think no serious Wireshark
> user will use an earlier version of OsX than 10.6, so we should be safe.
>
> (personal note, my old MacBook Pro has two cores and no hyper-thread, so
> personally I'm fine with 3 make jobs [note that this was the default in
> the OsX build script for a long time])
I'm using "make -j -l 2" (keep spawning jobs until the system load is
"2") on the buildbots. After a bit of experimentation that seems to
utilize available cores without overloading the system, particularly if
you have multiple builds running.
