I'm a student doing some research on Hyper-threading recently. I'm a little confused about the feature - L1 Data Cache Context Mode.
In the architecture optimization manual, it was described that L1 cache can operate in two modes:
The first level cache can operate in two modes depending on a context-ID bit:
Shared mode: The L1 data cache is fully shared by two logical processors.
Adaptive mode: In adaptive mode, memory accesses using the page directory is mapped identically across logical processors sharing the L1 data cache.
However, I am curious about how cache get partitioned in the adaptive mode according to the description.
On Intel arch, a value of 1 of
L1 Context ID indicates the L1 data cache mode can be set to either adaptive mode or shared mode, while a value of 0 indicates this feature is not supported. Check the definition of
IA32_MISC_ENABLE MSR Bit 24 (L1 Data Cache Context Mode) for details.
According to Intel® 64 and IA-32 Architectures Developer's Manual: Vol. 3A (Chapter 11/Cache Control), which I quote below:
In shared mode, the L1 data cache is competitively shared between logical processors. This is true even if the logical processors use identical CR3 registers and paging modes. In shared mode, linear addresses in the L1 data cache can be aliased, meaning that one linear address in the cache can point to different physical locations. The mechanism for resolving aliasing can lead to thrashing. For this reason, IA32_MISC_ENABLE[bit 24] = 0 is the preferred configuration for processors based on the Intel NetBurst microarchitecture that support Intel Hyper-Threading Technology.
Adaptive mode facilitates L1 data cache sharing between logical processors. When running in adaptive mode, the L1 data cache is shared across logical processors in the same core if:
• CR3 control registers for logical processors sharing the cache are identical.
• The same paging mode is used by logical processors sharing the cache.
In this situation, the entire L1 data cache is available to each logical processor (instead of being competitively shared). If CR3 values are different for the logical processors sharing an L1 data cache or the logical processors use different paging modes, processors compete for cache resources. This reduces the effective size of the cache for each logical processor. Aliasing of the cache is not allowed (which prevents data thrashing).
I just guess there is no definite approach for partitioning the L1 data cache.
The document just states that if you use the adaptive mode and if CR3 or the paging mode differ between cores, the cache is not shared and the cores "compete" for the cache. It doesn't tell how the partitioning works.
The most straightforward manner to implement this would be to statically reserve half of the ways of the data cache to each of the processors. This would essentially assign half the data cache statically to each processor.
Alternatively they could add an additional bit to the virtual tag of each cache line to distinguish which processor the line belongs to. This would allow a dynamic partition of the cache. This fits the description of "competing" for the cache better than a static partition.
If you really need to know, you could design some micro-benchmarks to verify that one these schemes is actually used.