Re: PARALLEL_MAKE

[Denys Dmytriyenko <denis@denix.org>]

On Fri, Mar 13, 2009 at 04:27:08PM +0100, Ulf Samuelsson wrote:
>>>> On the dual dual-core (4 total) boxes I have, I do -j6/4 threads.  On my
>>>> new quad-core box I haven't yet had a chance to figure out what's best.
>>>> I also use -j6/4 threads on an older dual-core amd64 box we've got
>>>> around and that seems best for my tasks at least (I tried a few
>>>> combinations of more jobs/threads and less and both were slower).
>>>>
>>>
>>> I looked at a few recipies which by a stanrge coincidence had
>>> PARALLEL_MAKE="",
>>
>> That's not a strange coincidence, that means a specific package has issues
>> building in parallel - usually some dependencies go out of order.
>>
>
> The strange coincidence was that *ALL*  the packages I *looked* at
> had PARALLEL_MAKE="".
> I made the erroneous assumption that most packages were this way.

Ah, I see. I'm not sure what is the percentage of "broken" packages...

>>> When, I rechecked, I had
>>> PARALLEL_MAKE ="-j 16"
>>> as well and this should be
>>> PARALLEL_MAKE ="-j16"
>>
>> Having only 4 cores and setting number of parallel makes to 16 kills your
>> performance, as system spends more time switching contexts instead of 
>> actually
>> doing something useful - i.e. compiling.
>
> The Core i7 has 4 cores and each core has two virtual CPUs so the system 
> sees 8 CPUs.
> I saw some commone from someone claiming that if
> you have n CPUs, then Buildroot should use make -j`2*n`.
>
> I thought about this, and came up with the following explanation.
> If you have one job per CPU, then  whenever you have to wait for the 
> harddisk, you stall this CPU.
> If you have several jobs per CPU, then you can gain some time
> by running the second job while the first job was waiting for the hard 
> disk.
>
> Whether this works in practice, I cannot say, but it would be interesting
> to have some real proof.

First of all, I believe -j`2*n` is very agressive. I've seen other suggestions 
of using -j`n+1` or -j`n+2`.
Second, your Core i7 still has 4 real cores. Even though HyperThreading 
presents each core as 2 virtual ones, those are not real 8 cores. And 
according to [1], sometimes using HyperThreading may degrade the overall 
performance...

So, you should experiment with your settings and find a sweet spot, which 
maybe somewhere between 4 and 16 threads :)

[1] http://en.wikipedia.org/wiki/Hyper-threading

-- 
Denys