VM: cache line size?

[Vladimir Kozlov]

 > 0. Is this the method one should use to determine the cache line size?

Depending for what use.

 > 1. Shouldn't that method be called cache_line_size()?

It is intentionally called prefetch_data_size() because it could be not 
accurate (different cache levels may have different cache line sizes, 
and on Sparc it is even worser). Which is fine for data prefetching 
since it is only affect performance and not correctness. But if you want 
the exact size you may need to add new functionality.

 >  2. Is that correct at this point that we default to 32-byte cache
 >     lines on x86? Should we instead default for 64?

It is fine since current code does not rely on precise value.

Regards,
Vladimir

On 11/21/12 10:52, Martijn Verburg wrote:
> Hi Aleksey,
>
> Disclaimer: "I have *not* dived deeply into this".  SOme thoughts inline.
>
>     Hi,
>
>     I need the VM to get the info about the cache line size on the current
>     system. Looking around the Hotspot code, I had found the usages like
>     this:
>          intx cache_line_size = prefetch_data_size();
>
>     ...where:
>
>     vm_version_sparc.hpp:
>        static intx prefetch_data_size()  {
>          return is_T4() ? 32 : 64;  // default prefetch block size on sparc
>        }
>
>     vm_version_x86.hpp:
>        static intx prefetch_data_size()  {
>          intx result = 0;
>          if (is_intel()) {
>            result = (_cpuid_info.dcp_cpuid4_ebx.bits.L1_line_size + 1);
>          } else if (is_amd()) {
>            result = _cpuid_info.ext_cpuid5_ecx.bits.L1_line_size;
>          }
>          if (result < 32) // not defined ?
>            result = 32;   // 32 bytes by default on x86 and other x64
>          return result;
>        }
>
>     At this point, I have a few questions:
>       0. Is this the method one should use to determine the cache line size?
>
>
> Not sure, but it seems that way after a quick spelunk through the code.
>
>       1. Shouldn't that method be called cache_line_size()?
>
>
> I would argue yes, *unless* there's a broader intent by the original
> author here. Perhaps reading multiple cache lines? Or reading data in a
> unit that isn't a cache line? But both of those seem unlikely :-).
>
>       2. Is that correct at this point that we default to 32-byte cache
>     lines
>     on x86? Should we instead default for 64?
>
>
> It wouldn't be safe to assume 64bytes. If you were running on a 32byte
> cache line based CPU (still common enough) and had this assumption then
> I'm not sure the results would be..... The other assumption is (32
> bytes) id probably safer although not really that safe if you consider
> the possibility of 16 byte cache line CPUs.
>
> I haven't heard of any exploding JVMs caused by this code so it might be
> that it's safe enough as it stands.
>
> Cheers,
> Martijn