RFR: 6515172: Runtime.availableProcessors() ignores Linux taskset command

[David Holmes]

Hi Thomas,

On 22/01/2016 7:29 PM, Thomas Stüfe wrote:
>
> On Fri, Jan 22, 2016 at 10:21 AM, Thomas Stüfe <thomas.stuefe at gmail.com
> <mailto:thomas.stuefe at gmail.com>> wrote:
>
>     Hi David,
>
>     I may be doing this wrong, but I do not get this to work for me
>     (ubuntu 14.4).
>
>     I built hs-rt with your patch. I am running a simple loop with
>     Runtime.availableProcessors(). I change affinity with taskset and
>     would expect output to change, but nothing changes.
>
>     java command line: ../images/jdk/bin/java -Xlog:os=trace test
>     taskset command:  taskset -p 0x1 <pid>
>
>     Output:
>
>     proc: 8
>     [73,525s][trace  ][os] active_processor_count: using static path -
>     configured processors: 8
>     [73,525s][trace  ][os] active_processor_count: sched_getaffinity
>     processor count: 8
>     proc: 8
>     ...
>
>     Kind Regards, Thomas
>
>
>
> I found that if I change:
>
>   if (sched_getaffinity(0, cpus_size, cpus_p) == 0) {
>
> to
>
>   if (sched_getaffinity(getpid(), cpus_size, cpus_p) == 0) {
>
> it works.
>
> Manpage on sched_getaffinity() states that "If pid is zero, then the
> mask of the calling process is returned." but seems that does not work.

Later manpages changed that to be "zero means the current thread" - see
the online manpage

http://man7.org/linux/man-pages/man2/sched_getaffinity.2.html

sched_getaffinity is really a thread-API that takes a pid_t - which is 
confusing to say the least. Regardless if you change the affinity of the 
process using taskset then it should affect all threads in the process, 
so this seems to be a glibc and/or kernel bug :(

I will see if I can get a C program that shows which threads in a 
process see the updated mask. This is yet another complexity to 
something that should be very simple, that I could really do without!

Many thanks for the report.

David
-----

> Kind Regards, Thomas
>
> ps.  I am happy you introduced the "os" log tag, now I can use this for
> AIX-specific logging without having to change shared files :-)
>
>
>
>
>     On Fri, Jan 22, 2016 at 9:06 AM, David Holmes
>     <david.holmes at oracle.com <mailto:david.holmes at oracle.com>> wrote:
>
>         First a special thanks to Martin Buchholz for his input,
>         feedback, critique and raising awareness of how non-simple this
>         issue is.
>
>         bug: https://bugs.openjdk.java.net/browse/JDK-6515172
>         webrev: http://cr.openjdk.java.net/~dholmes/6515172/webrev/
>
>         Basic problem:
>            processors available for use <= processors online <=
>         processors configured
>
>         but we always returned the number of online processors.
>
>         Solution is simple in its basic form: use sched_getaffinity to
>         get the scheduling affinity mask and count the number of
>         available processors.
>
>         Details are complicated by the desire to handle very large
>         processor systems. See the bug report for lots of detailed
>         discussions and references.
>
>         Testing:
>           - new test that verifies behaviour when running under taskset
>           - diagnostic hook injection (UseNewCodeN) to enable testing of
>         all code paths (one hook is left in for non-product to allow
>         easy testing of the dynamic path)
>           - JPRT
>
>         Compatability issues:
>         - the system code we're using now is at least 5 years old so
>         distro's older than that (which are not officially supported)
>         may not work
>         - anyone already running under a processor constrained
>         environment (like Docker) and using availableProcessor() to
>         "size" things, will find that size has now changed. We do not
>         expect this to be a problem - on the contrary we expect Docker
>         users to want the new behaviour.
>
>         Thanks,
>         David
>
>
>