NAME

lat_mem_rd - memory read latency benchmark

SYNOPSIS

lat_mem_rd [ -P <parallelism> ] [ -W <warmups> ] [ -N <repetitions> ] size_in_megabytes stride [ stride stride... ]

DESCRIPTION

lat_mem_rd measures memory read latency for varying memory sizes and strides. The results are reported in nanoseconds per load and have been verified accurate to within a few nanoseconds on an SGI Indy.

The entire memory hierarchy is measured, including onboard cache latency and size, external cache latency and size, main memory latency, and TLB miss latency.

Only data accesses are measured; the instruction cache is not measured.

The benchmark runs as two nested loops. The outer loop is the stride size. The inner loop is the array size. For each array size, the benchmark creates a ring of pointers that point backward one stride. Traversing the array is done by

p = (char **)*p;

in a for loop (the over head of the for loop is not significant; the loop is an unrolled loop 100 loads long).

The size of the array varies from 512 bytes to (typically) eight megabytes. For the small sizes, the cache will have an effect, and the loads will be much faster. This becomes much more apparent when the data is plotted.

Since this benchmark uses fixed-stride offsets in the pointer chain, it may be vulnerable to smart, stride-sensitive cache prefetching policies. Older machines were typically able to prefetch for sequential access patterns, and some were able to prefetch for strided forward access patterns, but only a few could prefetch for backward strided patterns. These capabilities are becoming more widespread in newer processors.

OUTPUT

Output format is intended as input to xgraph or some similar program (we use a perl script that produces pic input). There is a set of data produced for each stride. The data set title is the stride size and the data points are the array size in megabytes (floating point value) and the load latency over all points in that array.

INTERPRETING THE OUTPUT

The output is best examined in a graph where you typically get a graph that has four plateaus. The graph should plotted in log base 2 of the array size on the X axis and the latency on the Y axis. Each stride is then plotted as a curve. The plateaus that appear correspond to the onboard cache (if present), external cache (if present), main memory latency, and TLB miss latency.

As a rough guide, you may be able to extract the latencies of the various parts as follows, but you should really look at the graphs, since these rules of thumb do not always work (some systems do not have onboard cache, for example).

onboard cache: Try stride of 128 and array size of .00098.
external cache: Try stride of 128 and array size of .125.
main memory: Try stride of 128 and array size of 8.
TLB miss: Try the largest stride and the largest array.

BUGS

This program is dependent on the correct operation of mhz(8). If you are getting numbers that seem off, check that mhz(8) is giving you a clock rate that you believe.

ACKNOWLEDGEMENT

Funding for the development of this tool was provided by Sun Microsystems Computer Corporation.

AUTHOR

Carl Staelin and Larry McVoy

Comments, suggestions, and bug reports are always welcome.