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From Colm MacCarthaigh <>
Subject Re: Is async the answer
Date Fri, 18 Jan 2008 19:20:23 GMT
On Fri, Jan 18, 2008 at 01:52:02PM -0500, Akins, Brian wrote:
> On 1/18/08 12:18 PM, "Colm MacCarthaigh" <> wrote:
> > Hmmm, it depends what you mean by scale really. Async doesn't help a
> > daemon scale in terms of concurrency or throughput, if anything it might
> > even impede it, but it certainly can help improve latency and
> > responsivity greatly. On the whole, it's easy to see how it might make
> > the end user experience of a very busy server much more pleasant.
> I also wonder is that has actually been tested or if it's just a "factoid"?

I've tested, and it met my expectations on Linux 2.6 on Itanium, but I
can't guarantee that the experiments were free from my own bias I guess. 

> >> Response time never increased in any measurable amount.
> > 
> > I suspect it might though if the scheduler became bound, async would
> > route the interupts more efficiently.
> But, I wonder if the scheduler would become bound in a "reasonable" amount
> of traffic.

I think so, in some environments anyway. If you have a server tuned for
high throughput accross large bandwidth-delay product links then you
have the general problem of equal-priority threads sitting around with
quite a lot of large impending writes. Having them all in the polling
loop is inefficient, and async is going to reduce the latency a little,
though granted these days we may be talking about nanoseconds. And
I guess responsivity and high BDP don't go together anyway, due to the
speed of light.

> > The scalability wars should really be over,
> > everyone won - kernel's rule :-)
> Which is why I hate to see a ton of work go into async core if it actually
> does very little to help performance (or if it hurts it) and makes writing
> modules harder.  It braindead simple nowadays to write well behaved high
> performance modules (well, mostly) bcs you rarely worry about threads,
> reads/writes, etc.  Full async programming is just as challenging as
> handling a ton of threads yourself.

I think if it interests people and they want to work on it, cool stuff,
but don't neccessarily expect any actual pay-off in terms of
performance. One of the great things about an open source project is
that sometimes what gets worked on isn't driven by considerations other
than what people feel like working on. 

I'd be less worried about the effect on modules, many module authors 
already can't be bothered to make their modules thread-safe, but 
prefork still exists (and scales quite well, on many platforms).

Colm MacCárthaigh                        Public Key:

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