Andrew morton wrote:
>
> Trond Myklebust <[email protected]> wrote:
> >
> > On Thu, 2006-02-23 at 15:35 -0500, Bryan Fink wrote:
> > > Hi All. I'm running into a bit of trouble with NFS on 2.6. I see that
> > > at least Trond thought, mid-January, that "The readahead algorithm has
> > > been broken in 2.6.x for at least the past 6 months." (
> > > http://www.ussg.iu.edu/hypermail/linux/kernel/0601.2/0559.html) Anyone
> > > know if that has been fixed?
> >
> > No it hasn't been fixed. ...and no, this is not a problem that only
> > affects NFS: it just happens to give a more noticeable performance
> > impact due to the larger latency of NFS over a 100Mbps link.
>
> iirc, last time we went round this loop Ram and I were unable to reproduce it.
>
> Does anyone have a testcase?
Afaics, this problem was resolved a long ago.
The patch below should fix this problem. Does it?
Andrew, I'll resend it with a proper changelog and comments on Sunday,
currently I can't even do a compile test. I verified this patch still
applies cleanly.
------------------------------------------------------------------------------
>From - Thu Aug 4 20:33:03 2005
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Message-ID: <[email protected]>
Date: Thu, 04 Aug 2005 20:33:03 +0400
From: Oleg Nesterov <[email protected]>
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To: Ram Pai <[email protected]>,
Trond Myklebust <[email protected]>,
Linus Torvalds <[email protected]>,
Steven Pratt <[email protected]>, Andrew Morton <[email protected]>
Subject: Re: Readahead algorithm problems again...
References: <[email protected]> ...
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Oleg Nesterov wrote:
>
> What do you think about this patch?
Ohh... Sorry, I attached the wrong one.
--- 2.6.13-rc4/mm/readahead.c~ Thu Apr 7 12:59:41 2005
+++ 2.6.13-rc4/mm/readahead.c Thu Aug 4 20:25:14 2005
@@ -57,8 +57,8 @@ static inline void ra_off(struct file_ra
ra->start = 0;
ra->flags = 0;
ra->size = 0;
+ ra->ahead_size += ra->ahead_start;
ra->ahead_start = 0;
- ra->ahead_size = 0;
return;
}
@@ -423,8 +423,8 @@ static int make_ahead_window(struct addr
* congestion. The ahead window will any way be closed
* in case we failed due to excessive page cache hits.
*/
+ ra->ahead_size += ra->ahead_start;
ra->ahead_start = 0;
- ra->ahead_size = 0;
}
return ret;
@@ -507,7 +507,7 @@ page_cache_readahead(struct address_spac
if (ra->ahead_start == 0) { /* no ahead window yet */
if (!make_ahead_window(mapping, filp, ra, 0))
- goto out;
+ goto recheck;
}
/*
* Already have an ahead window, check if we crossed into it.
@@ -520,6 +520,9 @@ page_cache_readahead(struct address_spac
ra->start = ra->ahead_start;
ra->size = ra->ahead_size;
make_ahead_window(mapping, filp, ra, 0);
+recheck:
+ ra->prev_page = min(ra->prev_page,
+ ra->ahead_start + ra->ahead_size - 1);
}
out:
------------------------------------------------------------------------------
There is another one, from Steven Pratt:
------------------------------------------------------------------------------
>From - Sat Aug 13 11:49:43 2005
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To: Andrew Morton <[email protected]>
Cc: Ram Pai <[email protected]>, [email protected],
[email protected], [email protected]
Subject: Re: Readahead algorithm problems again...
References: <[email protected]> ...
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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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