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A Few Subtle Insights About UCP

Moinuddin K. Qureshi

Work on UCP done while at:

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First Things First

I thank Xing and Rajeev for:

1. Validating that UCP (based on misses) works2. Re-Validating that UCP (based on IPC) is slightly better

than one based on misses: ~1%-3%

As mentioned, this is not the 1st (or 2nd, or 3rd, or 4th …) paper to provide this insight

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Critique 1: UCP(MPKI) = UCP

Consider two apps, A and B, with identical miss rate curves

Num Ways in 4-way Cache0 1 2 3 4

MPK

I

10

5

4

3

21UCP(MPKI) gives 2 ways to both: A&B

A & B both access cache 1 per 100 inst, Cache Hit: 1 Cycle, Memory: 100 cycles

A has 99 integer ops (1 cycle each): CPI_A = (99+1+ MissRatePerc)/100

B has 99 FP ops (10 cycles each): CPI_B = (990+1+ MissRatePerc)/100

UCP(MPKC) 4 ways to A: IPC_best, WS_best

UCP(MICRO’06) optimizes perf more than UCP(MPKI)

Num Ways in 4-way Cache0 1 2 3 4

IPC

1.0

0.5

A

B

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Critique 2: Dynamic can beat Static Optima

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Critique 3: Not all Misses are Created Equal

CPI

MPK

I

Problem with Linear CPI Model of Xing

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UCP: The last 4.5 years …

Things I would have liked to see in literature: 1. Non-Integer Way Partition2. Utility Based Cache Insertion 3. Prefetch Aware Cache Partition

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Extension 1: Probabilistic Way Partition

Common criticism of way partitioning: We can only allocateInteger number of ways

A simple way to avoid this is Probabilistic Way Partition.

Say you want to allocate 3.5 ways to application A

Then on a cache miss, consult a Rand number generator If Randval > 50% of Randmax, then A gets 4 ways, else 3 ways

On average, A will end up getting 3.5 ways in the cache

Can go finer, say we want to allocate 4.125 ways to B

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Extension 2: Utility Based Cache Insertion

One can achieve the effect of partitioning by intelligent insertion

In a 16-way cache, a given application A can insert at 16 locations

If N applications share the cache the decision space is 16N

An efficient hardware scheme that obtains the best decision in this decision space will outperform both UCP and TADIP

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Extension 3: Prefetch Aware Partitioning

How does one do partitioning under prefetching ?

For applications whose dataset is prefetchable, we mayNot want to give cache space (even if it has high utility)

In-fact sometimes it’s a win-win to give more cache to irregularApps, as it provides more bandwidth available for prefetching

What is the right way to extend UCP to prefetches ?

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Summary

UCP: Partitioning based on misses works (simple)

Several work has shown UCP based on IPC works slightly better

There are several extensions of UCP still unexplored:-- Let me know if you are interested in exploring

questions/comments: moinqureshi@gmail.com