Turn on UseNUMA by default when prudent

Jesper Wilhelmsson jesper.wilhelmsson at oracle.com
Wed May 30 20:14:54 UTC 2012

On 2012-05-30 20:41, Igor Veresov wrote:
> Actually UseNUMA should already do what you want. Even if specified on the command line it will switch itself off if there's only one node present.

So, will setting UseNUMA to true as default be a platform independent way of 
solving this?

> igor
> On May 30, 2012, at 12:27 AM, Thomas Schatzl wrote:
>> 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
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