topic What are LLC hardware prefetches? in Software Tuning, Performance Optimization & Platform Monitoring

What are LLC hardware prefetches?

perfwise — Wed, 07 Dec 2011 20:31:45 GMT

Hi,

Reading documentation about L2_TRANS:

http://software.intel.com/sites/products/documentation/hpc/amplifierxe/en-us/lin/ug_docs/reference/snb/events/l2_trans.html

and OFFCORE_REQUESTS:

http://software.intel.com/sites/products/documentation/hpc/amplifierxe/en-us/lin/ug_docs/reference/snb/events/offcore_requests.html

A distinction is made about L2 hardware prefetches and LLC hardware prefetches.

Can someone please explain to me what a LLC hardware prefetch is, how does it differ from an L2 hardware prefetch?

All L2 hardware prefetches have to install at least in the L3 because the L3 is inclusive, correct? Is an LLC hardware prefetch a hardware prefetch generated by the L2 but that is only installed into the L3 and not also into the L2?

Thanks..

perfwise

What are LLC hardware prefetches?

Patrick_F_Intel1 — Fri, 09 Dec 2011 15:51:40 GMT

Hello Perfwise,
I'm researching this question.
Get back to you when I have an answer.
Pat

What are LLC hardware prefetches?

Patrick_F_Intel1 — Tue, 20 Dec 2011 15:32:04 GMT

Hello perfwise,
The 'LLC hardware prefetch' is a prefetch issued by the L2 prefetcher with a hint to fetch the data into the LLC, not the L2.
So... for your questions:
1) "All L2 hardware prefetches have to install at least in the L3 because the L3 is inclusive, correct?"
True.
2) "Is an LLC hardware prefetch a hardware prefetch generated by the L2 but that is only installed into the L3 and not also into the L2?"
True.

Pat