Caches

All MemHierarchy caches, regardless of level or type (shared, private, inclusive, noninclusive, etc.) use the same component, memHierarchy.Cache which is implemented in the SST::MemHierarchy::CacheController class. This picture shows the component structure. The light blue boxes show the available subcomponent slots. The cache has a number of ports which are not shown; it is recommended instead to use the cpulink and memlink subcomponents which have their own ports.

coherence

This subcomponent handles incoming events and sends outgoing ones according to the configured coherence protocol (or lack thereof). The cache component decides which coherence subcomponent to load depending on the parameters L1, coherence_protocol, and cache_type. Users should not explicitly load a subcomponent into this slot in the SST input file. The table below shows different combinations of parameters and which subcomponent gets loaded. Any combination of parameters not listed (for example, L1=true, coherence_protocol="mesi" and cache_type="noninclusive") is an invalid combination.

L1	coherence_protocol	cache_type	SubComponent
true	mesi, msi	inclusive	coherence.mesi_l1
true	none	inclusive	coherence.incoherent_l1
false	mesi, msi	inclusive	coherence.mesi_inclusive
false	mesi, msi	noninclusive	coherence.mesi_private_noninclusive
false	mesi, msi	noninclusive_with_directory	coherence.mesi_shared_noninclusive
false	none	noninclusive	coherence.incoherent

replacement

This subcomponent implements the cache replacement policy. It can be loaded by the user in the input file (e.g., setSubComponent("replacement", "replacement.mru")), otherwise it will be inferred from the replacement_policy cache parameter. A number of policies are included in memHierarchy but custom ones from other libraries can also be used by loading them in the input file. If the cache coherence configuration uses multiple cache arrays (e.g., a non-inclusive cache and directory), multiple replacement policies can be loaded in the slot.

replacement.lru

LRU is a least-recently-used policy. The timestamp of the most recent access to each line in a cache set is tracked and the line with the oldest timestamp is replaced.

replacement.lru-opt

LRU-opt is similar to LRU (replacement.lru) but is optimized to reduce coherence traffic in shared inclusive caches. If a line is unshared or unowned, it is preferred for replacement regardless of the LRU timestamp. Two lines with similar shared/owned status will use the LRU timestamp to determine the victim line.

replacement.lfu

This policy implements a least-frequently-used algorithm to replace blocks. Each access $x$ updates block $A$'s replacement weight ($weight_{A,x}$) as follows. $weight$ increments by 1000 each time a line in a set is accessed. The increment creates space between different lines' $weight_{A,x}$ values.

$$weight_{A,x} = (accesses_{A,x-1}*weight_{A,x-1} + weight)/(accesses_{A,x-1} + 1)$$ $$accesses_{A,x} = accesses_{A,x-1} + 1$$ $$weight = weight + 1000$$

replacement.lfu-opt

This policy is similar to replacement.lfu but is optimized to reduce coherence traffic in shared inclusive caches. To that end, the policy will prefer to evict a block that is unowned and unshared, but, given two blocks with similar sharing/owned status, will fall back to using the $weight_x$ as defined above to determine the victim line.

replacement.mru

This policy implements a most-recently-used algorithm to determine which line in a set should be replaced. The timestamp of the most recent access to each line is compared and the most-recently accessed line is replaced.

replacement.mru-opt

This policy is similar to replacement.mru but is optimized to reduce coherence traffic in shared inclusive caches. To that end, the policy will prefer to evict a block that is unowned and unshared, but, given two blocks with similar sharing/owned status, will fall back to using the most-recently-used timestamps to determine the victim line.

replacement.random

The random replacement policy randomly selects one of the blocks in the cache set to replace. It uses SST's Marsaglia generator.

replacement.nmru

This is a not-most-recently-used policy. Only the most recently used line in each set is tracked. A victim is randomly selected from the remaining lines. NMRU uses the Marsaglia random number generator.

hash

The hash slot loads a function that is used to map physical addresses to lines in the cache array. The subcomponent can be set in the input file or can be controlled using the cache's hash parameter. The only way to pass parameters to the hash subcomponent is to load the subcomponent in the SST input file and add parameters to it. By default, the cache loads a pass-through subcomponent that does not hash addresses.

hash.none

This subcomponent does not do any manipulation of the address passed to it. It is the default function used in the cache if none is specified.

$$addr_{out} = addr_{in}$$

hash.linear

This function does a linear hash of the address as shown.

$$addr_{out} = 1103515245*addr_{in} + 12345$$

hash.xor

This function does a simple XOR hash, XOR'ing each byte, $b_i$ in the input address with the next byte $b_{i+1}$, to obtain an output.

$$b_{out,i} = b_{in,i} \oplus b_{in,i+1}$$

prefetcher

Loading a prefetcher is optional. When loaded, the cache will notify the prefetcher of every cache access and its result (hit or miss). The prefetch subcomponent can then send prefetch requests to the cache. Prefetcher subcomponents are implemented using the MemHierarchy::CacheListener interface. Currently, all SST-provided prefetchers reside in the cassini library.

The cache has three parameters related to prefetch performance. The first, prefetch_delay_cycles, causes each prefetch sent by the prefetcher subcomponent to be delayed by some number of cycles before being handled. This compensates for prefetchers that do not implement their own timing and can issue prefetches in the same cycle that they receive notification of a cache access. The second and third parameters, max_outstanding_prefetch and drop_prefetch_mshr_level, allow the cache to throttle the amount of traffic generated by a prefetcher. max_outstanding_prefetch limits the total number of prefetch misses allowed at one time. drop_prefetch_mshr_level allows the cache to drop a prefetch if handling it would cause the MSHR occupancy to rise above the specified level.

listener

The listener subcomponent is nearly identical to the prefetcher and uses the same CacheListener API. Both subcomponents receive notification of every request handled by the cache. The difference is that while the prefetcher can send requests to the cache, a listener cannot. Listeners can be used to snoop traffic, generate traces, etc. Multiple listeners can be loaded into this slot.

cpulink and memlink

The cpulink and memlink subcomponent slots each load a memory link manager subcomponent that controls how the cache connects to other memory system components. These are intended to be used instead of the cache ports. cpulink can be thought of as the link on which requests are received (typically, the link towards a CPU or other processing element) while memlink is the link towards the memory, or, where the cache sends requests if the cache cannot handle them itself. Both subcomponents use the MemHierarchy::MemLinkBase API. MemHierchy provides three subcomponents that can be loaded into these slots: MemLink, MemNIC, and MemNICFour.

The picture above shows the different ways to configure these subcomponent slots. A couple things to note:

• The cache generally expects requests from the cpulink and will forward requests that miss in the cache to the memlink.
• "Other Component" means any other memHierarchy component or a non-memHierarchy component that uses the memHierarchy SST::Interfaces::StandardMem interface (memHierarchy.standardInterface). Any time the cache links to one of these, the memHierarchy.MemLink subcomponent should be used. This includes when linking to memHierarchy.Bus.
• "Network" means any network model that implements the SST::Interfaces::SimpleNetwork interface. Any time the cache links to other components through a network, the memHierarchy.MemNIC (or memHierarchy.MemNICFour) subcomponent should be used.
• MemNIC (and MemNICFour) in turn load subcomponent(s) that implement the SST::Interfaces::SimpleNetwork interface.
• If the cache is only connected to other components through a network, put a MemNIC (or MemNICFour) in the cpulink slot and leave the memlink slot empty. Traffic that the cache would have sent to the memlink will be sent through the cpulink subcomponent.

MemLink

This is the default link manager, and is used when a cache connects directly to another component rather than over a network. Examples include connecting to the memHierarchy interface loaded by a CPU, another cache, or the memHierarchy Bus component.

MemNIC

MemNIC is used when the cache is connected to a network. If the cache has a single connection point into the network, the cpulink slot should contain the link manager and the memlink slot can be left empty.

MemNICFour

Like MemNIC, MemNICFour is used to connect the cache to a network. However, it has four separate ports for different coherence traffic classes (request, data, acknowledgement, forward). MemHierarchy does not require coherence classes to be separated for correctness and so either the MemNIC (single network) or MemNICFour (multiple networks) subcomponent can be used depending on the architecture to be modeled.