Request for review (XS): 8006431: os::Bsd::initialize_system_info() sets _physical_memory too large

[David Holmes]

Hi Bengt,

I must say the docs for these sysctl values is not very good. Nothing 
states that HW_USERMEM is 32-bit, and logically it makes more sense to 
use the amount of available memory for sizing things rather than the 
amount of physical memory. :(

It would seem that HW_USERMEM is newer than the "deprecated" 32-bit 
HW_PHYSMEM, so when was it introduced? If I go to the 10.4-intel version 
of the manpage you link there is no HW_MEMSIZE listed.

David

On 18/01/2013 4:36 PM, Bengt Rutisson wrote:
>
> Hi all,
>
> Could I have a couple of reviews for this small fix?
>
> http://cr.openjdk.java.net/~brutisso/8006431/webrev.00/
>
> On OSX we used HW_USERMEM value from sysctl() to get the available
> physical memory on the machine. This is a 32 bit value but we store it
> in a 64 bit variable. This means that we get kind of random and normally
> very large values for what we think the physical memory is.
>
> We actually don't want a 32 bit value since we want to handle machines
> with more than 2 or 4 GB of memory. Instead of HW_USERMEM we should be
> using HW_MEMSIZE which will give us a 64 bit value.
>
> See:
> https://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man3/sysctl.3.html
>
> Thanks Staffan Larsen for both detecting the problem and providing a
> solution.
>
> This is a one-word-change. So, to save you a mouse click on the webrev
> link above, here is the diff:
>
> --- a/src/os/bsd/vm/os_bsd.cpp
> +++ b/src/os/bsd/vm/os_bsd.cpp
> @@ -260,7 +260,7 @@
> * instead of hw.physmem because we need size of allocatable memory
> */
> mib[0] = CTL_HW;
> - mib[1] = HW_USERMEM;
> + mib[1] = HW_MEMSIZE;
> len = sizeof(mem_val);
> if (sysctl(mib, 2, &mem_val, &len, NULL, 0) != -1)
> _physical_memory = mem_val;
>
> Thanks,
> Bengt