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                                Ph.D. DEFENSE

WHERE:        University of Rochester
              Computer Science Department
              601 Computer Studies Building

WHEN:         January 18, 1993
              3:00 P.M.

SPEAKER:      CEZARY DUBNICKI
              University of Rochester


"The Effects of Block Size on the Performance of Coherent Caches in Shared-
                          Memory Multiprocessors"

Several studies have shown that the performance of coherent caches depends on
the relationship between the cache block size and the granularity of sharing and
locality exhibited by the program.  Large cache blocks exploit processor and
spatial locality, but may cause unnecessary cache invalidations due to false
sharing.  Small cache blocks can reduce the number of cache invalidations, but
increase the number of bus or network transactions required to load data into 
the cache.  In this dissertation, we use reference traces from a variety of 
parallel programs and detailed simulation of a scalable shared-memory multiprocessor to examine the effects of cache block size on the performance of coherent
caches and quantify this impact with respect to the network bandwidth and
latency.

Our results suggest that, regardless of the available bandwidth or latency,
applications with good spatial locality favor long cache lines, and for these
applications the relative benefits of longer cache lines increase with the
bandwidth and latency.  For those applications with poor spatial locality, the
best choice of cache line size is determined by the product of the network
bandwidth and latency, and for these applications, the performance penalty
induced by long cache lines increases as this product decreases.  We also
found that the performance penalty of a mismatch between the cache block size and the sharing patterns exhibited by applications increases with an increase in
latency, and decreases with an increase in bandwidth, and can be substantial
even on machines with infinite bandwidth.

To reduce this penalty, we propose a new cache organization that adjusts the
size of data blocks dynamically according to recent reference patterns.  In
this new cache organization, blocks are split in two when false sharing occurs,
and merged back together to exploit spatial locality.  Results of simulations
in which we varied both the network bandwidth and latency indicate that for
the suite of applications we consider, the adjustable block size cache organization performs better than every fixed block size alternative (including caches
that prefetch multiple lines).  In addition, the performance benefits of
adjustable block size caches increase with an increase in the ratio of network
latency to bandwidth.  We conclude that adjusting the block size in response
to reference behavior can significantly improve the performance of coherent
caches, especially when there is variability in the granularity of sharing
exhibited by applications.