Fast Pentium memcpy() corruption bug appears to be fixed (finally!)

Robert L Krawitz (rlk@tiac.net)
Sun, 3 Mar 1996 19:41:48 -0500

I believe that I have fixed the corruption problem with my fast
memcpy() routine. The fix was to force the routine to read the first
word from both the source and destination block with conventional
instructions first. I did this in the belief that the problem was due
to a page fault at the wrong time (since this only seemed to happen
when the machine was under significant load). My guess appears (so
far) to be correct. I ran my usual test (two separate cvs diff's of
the Linux kernel concurrently) without any failures. I repeated it
with 7 concurrent cvs diff's, and had no errors, either.

The patch and information may be found on my Linux web page
(http://www.tiac.net/users/rlk/linux.html). Summary Byte benchmarks
with and without the patch follow -- the complete results are on that
page.

With the patch (1.3.70):

INDEX VALUES
TEST BASELINE RESULT INDEX

Arithmetic Test (type = double) 2541.7 16716.9 6.6
Dhrystone 2 without register variables 22366.3 132050.4 5.9
Execl Throughput Test 16.5 788.8 47.8
File Copy (30 seconds) 179.0 5723.0 32.0
Pipe-based Context Switching Test 1318.5 20302.0 15.4
Shell scripts (8 concurrent) 4.0 36.0 9.0
=========
SUM of 6 items 116.7
AVERAGE 19.4

Without the patch (1.3.70 otherwise configured identically):

INDEX VALUES
TEST BASELINE RESULT INDEX

Arithmetic Test (type = double) 2541.7 16721.4 6.6
Dhrystone 2 without register variables 22366.3 132044.4 5.9
Execl Throughput Test 16.5 632.4 38.3
File Copy (30 seconds) 179.0 5263.0 29.4
Pipe-based Context Switching Test 1318.5 19444.4 14.7
Shell scripts (8 concurrent) 4.0 31.0 7.8
=========
SUM of 6 items 102.7
AVERAGE 17.1

-- 
Robert Krawitz <rlk@tiac.net>


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