Turn on UseNUMA by default when prudent

[Thomas Schatzl]

Hi all,

On Tue, 2012-05-29 at 21:56 +0200, Jesper Wilhelmsson wrote:
> Hi Eric,
> 
> As long as this is based on actual data and not just a hunch, I personally 
> think it is a good idea. I don't know if we have any policies about platform 
> specific optimizations like this though.
> 
> I have some comments on the code layout and there are a few typos, but I guess 
> this is still a draft so I won't pick on that right now.
> 
> One thing I wonder though is in os_linux_x86.cpp:
> 
> if (VM_Version::cpu_family() == 0x15 || VM_Version::cpu_family() == 0x10) {
> 
> Is this the only way to identify the proper processor family? It doesn't seem 
> very future proof. How often would you have to change this code to keep it up 
> to date with new hardware?

 just a question, if this is implemented, wouldn't it more prudent to
actually check whether the VM process runs on a NUMA machine, and
actually has its computing (or memory) resources distributed across
several nodes instead of a check for some arbitrary processors and
processor identifiers?

This would, given that the OS typically provides this information
anyway, also immediately support e.g. sparc setups. It also avoids
distributing memory when the user explicitly assigned the VM to a single
node...

>From memory, on solaris above mentioned detection works approximately as
follows:

  - detect the total amount of leaf locality groups (=nodes on Solaris)
in the system, e.g. via lgrp_nlgrps()
  - from the root node (retrieved via lgrp_root()), iterate over its
children and leaf lgroups via lgrp_children().
    - for each of the leaf lgroups found, check whether there is an
active cpu for this process in it using lgrp_cpus(); if so, increment
counter

Maybe there is a better way to do that though.

On Linux, numa_get_run_node_mask() may provide the same information when
called during initialization.
On Windows, it seems that a combination of GetProcessAffinityMask() and
GetNUMAProcessorNode() may be useful.
(From a cursory web search for the latter two; not sure about other
OSes, but you could simply provide a dummy for those)

I'd guess that some of the needed functionality to implement this is
already provided by the current Hotspot code base.


Ergonomics stuff is typically handled in runtime/arguments.?pp, so it
might be a better place as a location for updating globals than putting
this detection in some os-specific initialization code.

Eg.

  if (FLAG_IS_DEFAULT(UseNUMA)) {
    UseNUMA := [maybe some other conditions &&]
(os::get_num_active_numa_nodes() > 1);
  }

in e.g. Arguments::set_ergonomics_flags() or similar.

Seems a lot nicer than an explicit check for some processor family.
Maybe a little more work though.

Hth,
  Thomas