Re: Make pipe data structure be a circular list of pages, rather than

[Lee Revell <rlrevell@joe-job.com>]

On Fri, 2005-01-07 at 17:46 -0600, Chris Friesen wrote:
> Mike Waychison wrote:
> 
> > This got me to thinking about how you can heuristically optimize away
> > coalescing support and still allow PAGE_SIZE bytes minimum in the
> > effective buffer.
> 
> While coalescing may be a win in some cases, there should also be some 
> way to tell the kernel to NOT coalesce, to handle the case where you 
> want minimum latency at the cost of some throughput.

Many latency critical apps use (tmpfs mounted) FIFO's for IPC; the Linux
FIFO being one of the fastest known IPC mechanisms.  Each client in the
JACK (http://jackit.sf.net) graph wakes the next one by writing a single
byte to a FIFO.  Ardour's GUI, control, and audio threads interact via a
similar mechanism.  How would you expect this change to impact the inter
thread wakeup latency?  It's confusing when people say "performance",
meaning "increased throughput albeit with more latency".  For many
people that's a regression.

These apps *certainly* care about performance, they just don't define it
in terms of throughput.

And yes we do know futexes are the right tool for this but they weren't
available at the time and aren't available on 2.4.

Lee