Re: Approaches to making io_submit not block

["Ted Ts'o" <tytso@mit.edu>]

On Thu, Sep 01, 2011 at 04:54:15PM +1000, Dave Chinner wrote:
> Apples to oranges - there's orders of magnitude of difference in the
> number of operations that the different stacks do. Allocation in XFS
> when it does not block can still take milliseconds of CPU time; in
> comparison, the networking stack is expected to process thousands of
> packets in that same time frame.  IOWs, the scale of processing per
> item of work is -vastly- different - that's why working in process
> context matters a great deal to the networking stack but not to
> allocation in XFS.

That may be true for hard drives, but PCIe attached flash can support
millions of IOP's per second --- i.e., at least hundreds of IOP's in
milliseconds.  Yes, these devices are expensive, but so are the
thousand-disk RAID arrays that some people attach via XFS.  :-)

There are people in the ext4 development community interested in
looking at such devices.  We've made some improvements, we (and by
this I mean the whole kernel) are a long, long way from supporting
such beasts properly....

							- Ted