Re: question about max_readahead for ide devices in 2.4?

[John Salmon]

>>>>> "Andrew" == Andrew Morton <akpm@xxxxxxxx> writes:

Andrew> John Salmon <jsalmon@xxxxxxxxxxxxxx> wrote:
>> 
>> 
>> 
>> Several "tuning" recommendations suggest that sequential accesses of
>> large files, and hence the performance of busy web servers, can be improved
>> by changing the maximum readahead value with, e.g.,
>> 
>> echo 511 > /proc/sys/vm/max-readahead
>> 
>> But it looks to me like get_max_readahead in filemap.c ignores the
>> value set by /proc/sys in favor of max_readahead[major][minor] whenever
>> max_readahead[major] is non-NULL.  And furthermore that 
>> max_readahead[major] IS initialized to non-NULL for ide devices in
>> init_gendisk.  (N.B. I'm looking at 2.4 sources).
>> 
>> Conclusion: echoing a value into /proc/sys/vm/max-readahead won't change the
>> readahead behavior for already-probed IDE devices.
>> 
>> Is this correct, or am I missing something?

Andrew> That's correct - it's all a bit weird.   You should use

Andrew> 	blockdev --setra 511 /dev/hda

Andrew> for IDE devices.  Not sure about scsi.  You may as well set
Andrew> /proc/sys/vm/max-readahead to the same thing.

Are you sure?  It looks like that invokes the BLKRASET ioctl, which
sets an entry in the read_ahead[MAJOR(dev)] array.  But the only code
that uses the read_ahead[] array is in fs/hfs/file.c, and I'm definitely
not using an hfs filesystem.

Or am I missing something else??

Thanks,
John Salmon

P.S.  The weird thing is that I tried the blockdev --setra suggestoin
and it *seemed to* improve performance.  This is why drug tests are
double-blind ... the patient (the computer) may not be susceptible to
the placebo effect, but the doctor (me) may be :-(.


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