Hypothetical progams might desire cross-CPU coalescing of IO

Posted Jun 7, 2013 2:27 UTC (Fri) by dlang (guest, #313)
In reply to: Hypothetical progams might desire cross-CPU coalescing of IO by ejr
Parent article: The multiqueue block layer

when you have things like databases where many applications are accessing one file, how common is it for the different applications to be making adjacent writes at the same time?

It may happen, but it's not going to be the common case.

Hypothetical progams might desire cross-CPU coalescing of IO

Posted Jun 7, 2013 10:06 UTC (Fri) by ejr (subscriber, #51652) [Link] (1 responses)

In my world (HPC), having multiple coordinated processes accessing slices of a range *is* the common case. We have to fight for anything else to have even reasonable performance support. See lustre.

But this case often is better handled outside the OS. There could be different interface where clients post their read buffers and the OS fills them in some hardware-optimal order, but that's been considered for >20 years and has no solution yet.

Hypothetical progams might desire cross-CPU coalescing of IO

Posted Jun 7, 2013 18:18 UTC (Fri) by dlang (guest, #313) [Link]

you are processing slices of a range, but are you really writing the adjacent slices at almost exactly the same time from multiple processes? that's what it would take to give the OS the chance to combine the output from the different processes into one write to disk.

Also, in HPC aren't you dealing with many different systems accessing your data over the network rather than multiple processes on one machine?

What we are talking about here is the chance that things running on different CPUs in a single system are generating disk writes that the OS could combine into a single write before sending it to the drives

for reads this isn't as big a deal, readahead should go a long way towards making the issue moot.