Reality check time. Quoting Marc from the beginning of the thread:
> I tested the following under linux-2.4.8-ac8, linux-2.4.8pre4 and
> 2.4.5pre4, all had similar behaviour.
2.4.5pre4 has drop-behind and so does -ac8. Still, if the readahead window
is too large then drop-behind isn't going to help a lot.
Let's test the idea that readahead is the problem. If it is, then disabling
readahead should make the lowlevel disk throughput match the highlevel
throughput. Marc, could you please try it with this patch:
--- ../2.4.9.clean/mm/filemap.c Thu Aug 16 14:12:07 2001
+++ ./mm/filemap.c Sun Aug 26 02:24:50 2001
@@ -886,6 +886,7 @@
raend = filp->f_raend;
max_ahead = 0;
+return;
/*
* The current page is locked.
-
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