--- trunk/reactos/lib/string/i386/memcpy_asm.s 2005-12-03 18:16:02 UTC (rev 19842)
+++ trunk/reactos/lib/string/i386/memcpy_asm.s 2005-12-03 19:40:52 UTC (rev 19843)
@@ -1,9 +1,7 @@
-/*
- * $Id$
- */
-
/*
* void *memcpy (void *to, const void *from, size_t count)
+ *
+ * Some optimization research can be found in media/doc/memcpy_optimize.txt
*/
.globl _memcpy
--- trunk/reactos/media/doc/memcpy_optimize.txt 2005-12-03 18:16:02 UTC (rev 19842)
+++ trunk/reactos/media/doc/memcpy_optimize.txt 2005-12-03 19:40:52 UTC (rev 19843)
@@ -0,0 +1,55 @@
+Surfing the Internet, I stumbled upon http://www.sciencemark.org where you
+can download a benchmark program that (amongst others) can benchmark different
+x86 memcpy implementations. Running that benchmark on my machine revealed that
+the fastest implementation was roughly twice as fast as the "rep movsl"
+implementation (lib/string/i386/memcpy_asm.s) that ReactOS uses.
+To test the alternate implementations in a ReactOS setting, I first
+instrumented the existing memcpy implementation to log with which arguments
+it was being called. I then booted ReactOS, started a background compile in it
+(to generate some I/O) and played a game of Solitaire (to generate graphics
+operations). After loosing the game, I shut down ReactOS. I then extracted
+the memcpy calls roughly between the start of Explorer (to get rid of one time
+startup effects) an shutdown. The resulting call profile is attached below.
+I then used that profile to make calls to the existing memcpy and an alternate
+implementation (I selected the "MMX registry copy with SSE prefetching"),
+taking care to use different source and destination regions to remove caching
+effects. The profile consisted of roughly 250000 calls to memcpy, I found
+that I had to execute the profile 10000 times to get "reasonable" time values.
+To compensate for the overhead of the test program, I also ran a test where
+the whole memcpy routine consisted of a single instruction: "ret". The test
+results, after applying a correction for the overhead:
+
+rep movl 70.5 sec
+mmx registers 58.3 sec
+Speed increase: 17%
+
+(Test machine: AMD Athlon MP 2800+ running Linux).
+Although the relative speed increase is nice (17%), we also have to look at the
+absolute speed increase. Remember that the 70.5 sec for the "rep movl" case
+was obtained by running the whole profile 10000 times. This means that all the
+memcpy's executed during the profiling run of ReactOS together took only
+0.00705 seconds. So the conclusion has to be that we're simply not spending
+a significant amount of time in memcpy (BTW, our memcpy implementation is
+shared between kernel and user mode, of the total of 250000 memcpy calls about
+90% were made from kernel mode and 10% from user mode), so optimizing memcpy
+(although possible) will not result in a significant better performance of
+ReactOS as a whole.
+Just for fun, I then used only the part of the profile where the memory area
+was larger than 128 bytes. The MMX implementation actually only runs for sizes
+over 128 bytes, for smaller sizes it deferred to the "rep movl" implementation.
+According to the profile, the vast majority of memcpy calls is made with a
+size smaller than 128 bytes (96.8%).
+
+rep movl 52.9 sec
+mmx registers 27.1 sec
+Speed increase 48%
+
+This is more or less in line with the results I got from the membench benchmark
+from http://www.sciencemark.org.
+
+Final conclusion: Although optimizing memcpy is useful (and feasible) for
+transfer of large blocks, the usage pattern in ReactOS consists mostly of
+small blocks. The resulting absolute spead increase doesn't justify the
+increased code complexity.
+
+2005/12/03 GvG
Property changes on: trunk/reactos/media/doc/memcpy_optimize.txt
___________________________________________________________________
Name: svn:eol-style
+ native