Re: Readahead algorithm problems again...

[Steven Pratt]

This is a multi-part message in MIME format.The current current get_init_ra_size is not optimal across different IO 
sizes and max_readahead values.  Here is a quick summary of sizes 
computed under current design and under the attached patch.  All of 
these assume 1st IO at offset 0, or 1st detected sequential IO.

32k max, 4k request

old         new
-----------------
8k        8k
16k       16k
32k       32k

128k max, 4k request
old         new
-----------------
32k         16k
64k         32k
128k        64k
128k       128k

128k max, 32k request
old         new
-----------------
32k         64k    <-----
64k        128k
128k       128k


512k max, 4k request
old         new
-----------------
4k         32k     <----
16k        64k
64k       128k
128k      256k
512k      512k


Steve

--- linux-2.6.12/mm/readahead.org.c	2005-08-01 08:52:12.000000000 -0500
+++ linux-2.6.12/mm/readahead.c	2005-08-10 10:16:52.000000000 -0500
@@ -72,10 +72,10 @@ static unsigned long get_init_ra_size(un
{
	unsigned long newsize = roundup_pow_of_two(size);

-	if (newsize <= max / 64)
-		newsize = newsize * newsize;
+	if (newsize <= max / 32)
+		newsize = newsize * 4;
	else if (newsize <= max / 4)
-		newsize = max / 4;
+		newsize = newsize * 2;
	else
		newsize = max;
	return newsize;

------------------------------------------------------------------------------

Oleg.
-
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