Hi Erik,
thanks for the explanation.
Regarding build times, the current heuristics scores ok on my high-end
machine (I get more or less same time as with JDK 8 build) - but with a
lower spec machine (i.e. laptop with dual core intel i5) it gets much
much worse - i.e. I used to be able to build in 7 minutes on my laptop
(using ccache) - now build time is at least double that figure.
I know it's an hard problem to decide how many cores to use but there
seem to be a pattern emerging:
* low-end machines get completely swamped by the build load
* CPU bound tests run into troubles when reusing same concurrency
settings, even on high-end hardware. Without playing with timeouts it's
impossible to get a clean test sheet.
* on relatively high-end HW, current build concurrency settings seem to
be doing ok.
Realistically, I believe anything that uses more than n/2 virtual
processors is going to face troubles sooner or later; the build might be
ok since there's so much IO going on (reading/writing files) - but the
more the build will become CPU intensive (and sjavac might help with
that) the more current settings could become a bottleneck.
Maurizio
On 14/09/15 17:05, Erik Joelsson wrote:
Hello,
When I implemented the heuristic to choose a suitable default
concurrency, I only ever worried about the build. I think having tests
use the same concurrency setting must be a new feature? In any case,
it seems like there is a case for reducing concurrency when running
tests.
Another note. It at least used to be quite tricky to get correct
information about cores vs hyperthreading from the OS. I know today we
aren't even consistent with this across platforms. Perhaps we should
revisit this heuristic and take hyperthreading into consideration too.
The current implemenation uses 100% of number of virtual cpus when 1
to 4 of them, then 90% at 5 to 16. After that it caps out at 16. (I
might remember some detail wrong here)
/Erik
On 2015-09-14 04:10, Maurizio Cimadamore wrote:
The information I posted was slightly incorrect, sorry - my machine
has 8 cores (and 16 virtual processors) - so you see why choosing
concurrency factor of 14 is particularly bad in this setup.
Maurizio
On 14/09/15 12:03, Maurizio Cimadamore wrote:
Hi,
I realized that the concurrency factor inferred by the JDK build
might be too high; on a 16 core machine, concurrency is set to 14 -
which then leads to absurd load averages (50-ish) when
building/running tests. High load when building is not a big issue,
but when running test this almost always turns into spurious
failures due to timeouts. I know I can override the concurrency
factor with --with-jobs - but I was curious as to why the default
parameter is set to such aggressive value?
Thanks
Maurizio
