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test/memcpy: fix optimistic results on Linux

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When executed on Linux, the test was impaired by the copy-on-write
optimisation since the source buffer was never initialised. By default,
Linux only maps a zeroed page until the first write access to the page
occurs. Since the source buffer was never written, the corresponding
page was always present in the physically-indexed data cache. In
consequence, the test merely measured write performance (similar to memset).

genodelabs/genode#4454
This commit is contained in:
Johannes Schlatow 2022-03-24 11:34:52 +01:00 committed by Christian Helmuth
parent 62f37c5b1b
commit 07736d1689
2 changed files with 20 additions and 5 deletions
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15
repos/libports/run/memcpy.run
View File

@ -72,7 +72,14 @@ exit 0
# Linux baseline measurements
#
# Raspberry Pi 1
# bytewise memcpy: copied 8388608 KiB in 93390210 usecs (87 MiB/sec)
# libc memcpy: copied 8388608 KiB in 6238602 usecs (1313 MiB/sec)
# libc memset: copied 8388608 KiB in 6023324 usecs (1360 MiB/sec)
# Zynq-7000 @ 666MHz (L2 prefetching enabled, offset=0)
# Genode memcpy: copied 8388608 KiB in 27362177 usecs (299 MiB/sec)
# bytewise memcpy: copied 8388608 KiB in 43882888 usecs (186 MiB/sec)
# libc memcpy: copied 8388608 KiB in 28702066 usecs (285 MiB/sec)
# libc memset: copied 8388608 KiB in 4033019 usecs (2031 MiB/sec)
# Zynq-7000 @ 666MHz (L2 prefetching enabled, offset=4)
# Genode memcpy: copied 8388608 KiB in 14985454 usecs (546 MiB/sec)
# bytewise memcpy: copied 8388608 KiB in 39478781 usecs (207 MiB/sec)
# libc memcpy: copied 8388608 KiB in 28792091 usecs (284 MiB/sec)
# libc memset: copied 8388608 KiB in 4041102 usecs (2027 MiB/sec)

10
repos/libports/src/test/memcpy/memcpy.h
View File

@ -16,11 +16,19 @@ void memcpy_test(void * dst = nullptr, void * src = nullptr,
void * const from_buf = src ? src : malloc(size);
void * const to_buf = dst ? dst : malloc(size);
/**
* initialising the buffer (with any value) is necessary to
* a) circumvent copy-on-write optimisation on linux and
* b) all pages are already allocated and mapped
*/
memset(from_buf, 0, size);
memset(to_buf, 0, size);
Test test;
test.start();
for (unsigned i = 0; i < ITERATION; i++)
test.copy(to_buf, from_buf, BUF_SIZE);
test.copy(to_buf, from_buf, size);
test.finished();