Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

Michael K. Edwards
2007-02-22T01:04:52Z

On 2/21/07, Ingo Molnar <mingo@elte.hu> wrote:
> threadlets (and syslets) are parallel contexts and they behave so -
> queuing and execution semantics are then ontop of that, implemented
> either by glibc, or implemented by the application. There is no
> 'pipeline' of requests imposed - the structure of pending requests is
> totally free-form. For example in threadlet-test.c i've in essence
> implemented a 'set of requests' with the submission site only interested
> in whether all requests are done or not - but any stricter (or even
> looser) semantics and ordering can be used too.

In short, you have a dataflow model with infinite parallelism,
implemented using threads of control mapped willy-nilly onto the
underlying hardware. This has not proven to be a successful model in
the past.

> in terms of AIO, the best queueing model is i think what the kernel uses
> internally: freely ordered, with barrier support. (That is equivalent to
> a "queue of sets", where the queue are the barriers, and the sets are
> the requests within barriers. If there is no barrier pending then
> there's just one large freely-ordered set of requests.)

That's a big part of why Linux scales poorly for workloads that
involve a large volume of in-flight I/O transactions. Unless you
essentially lock one application thread to each CPU core, with a
complete understanding of its cache sharing and latency relationships
to all the other cores, and do your own userspace I/O scheduling and
dispatching state machine -- which is what all industrial-strength
databases and other sorts of transaction engines currently do -- you
get the same old best-effort context-thrashing scheduler we've had
since Solaris 2.0.

Let's do something genuinely better this time, OK?

Cheers,
- Michael