Re: 3x slower file reading oddity

[Andrew Morton]

dean gaudet wrote:
> 
> ...
> > You'll get best throughput with a single read thread.
> 
> what if you have a disk array with lots of spindles?  it seems at some
> point that you need to give the array or some lower level driver a lot of
> i/os to choose from so that it can get better parallelism out of the
> hardware.

mm.  For that particular test, you'd get nice speedups from striping
the blockgroups across disks, so each `cat' is probably talking to
a different disk.  I don't think I've seen anything like that proposed
though.

But regardless of the disk topology, the sanest way to get good IO
scheduling is to throw a lot of requests at the block layer.  That's
simple for writes.  But for reads, it's harder.

You could fork one `cat' per file ;)  (Not so silly, really.  But if
you took this approach, you'd need "many" more threads than blockgroups).

Or teach `cat' to perform asynchronous (aio) reads.  You'd need async
opens, too.   But generally we get a good cache hit rate against the
data which is needed to open a small file.

hmm.  What else?  Physical readahead - read metadata into the block
device's pagecache and flip pages from there into directories and
files on-demand.  Fat chance of that happening.

Or change ext2/3 to not place directories in different block groups
at all.  That's super-effective, but does cause somewhat worse long-term
fragmentation.

You can probably lessen the seek-rate by accessing the files in the correct
order.  Read all the files from a directory before descending into any of
its subdirectories.  Can find(1) do that?  You should be able to pretty
much achieve disk bandwidth this way - it depends on how bad the inter-
and intra-file fragmentation has become.

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