aio_read template.

[Volker Lendecke]

Hi, David!

On Wed, Jun 08, 2005 at 08:49:49AM -0400, David Collier-Brown wrote:
>   Interesting!  
>   I had a similar problem about a year ago with I/O from a large 
> array, where either misordering or reads of ridiculous sizes coming
> from NT Media Server (of all the things that should try to stream reads!) 
> would result in pessimal throughput.
>  I found that a simple buffered read of the stripe width would
> make it optimal, and that had the side-effect of improving
> normal I/O from Solaris UFS and QFS.

When you look at the pattern XPSP2 gives you, this does not really work that
way, at least I don't know how to keep sanity. The following table shows the
read pattern I see coming from XPSP2 when the Win32 app reads 256k blocks, the
tables shows offset/length:

0 61440
61440 4096
65536 61440
126976 4096
131072 61440
192512 4096
196608 61440
258048 4096

Ok, so far it's sequential. But now something weird happens:

524288 61440
262144 61440
585728 4096
323584 4096
589824 61440
327680 61440
651264 4096
389120 4096
655360 61440
393216 61440
716800 4096
454656 4096
720896 61440
458752 61440
782336 4096
520192 4096

This looks like two threads reading in parallel, blocks starting at 256k and
512k. This happens for example with cygnus dd bs=256k which most probably does
not issue multi-threaded requests, as well as with the XDD tool from
ioperformance.com with explicit setting to sequentially read.

786432 61440
847872 4096
851968 61440
913408 4096
917504 61440
978944 4096
983040 61440
1044480 4096
1048576 61440
1114112 61440
1110016 4096
1175552 4096
1179648 61440
1241088 4096
1245184 61440
1306624 4096

Ok, this was 2 256k blocks sequentially, and we're back to parallel reads
again:

1572864 61440
1310720 61440
1634304 4096
1372160 4096
1638400 61440
1376256 61440
1699840 4096
1437696 4096
1703936 61440
1441792 61440
1765376 4096
1503232 4096
1769472 61440
1507328 61440
1830912 4096
1568768 4096

.... And sequential again (not shown anymore). This pattern proceeds until the
file is ended.

Now tell me how I can sanely detect this kind of pattern with a cache... :-)

De-install SP1 or use any other Windows version and you're fine.

Volker
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