Re: [boost] [Fibers] Performance

Subject: Re: [boost] [Fibers] Performance
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2014-01-22 18:55:57

On 22 Jan 2014 at 18:37, Bjorn Reese wrote:

> > Niall, would you be able to propose a more universal model? Please
> > read this as a simple invitation rather than a challenge. The goal of
> 
> A good place to start is to understand the limitations of the Asio
> model.

A great post Bjorn. You definitely explained it better than I. And my 
public thanks to you for all your work with me on improving AFIO 
(I'll be replying to your email soon, I ought to submit my maths 
coursework tomorrow).

I should elaborate on the importance of easily chaining operations 
into patterns as it's probably non-obvious. Storage i/o, unlike fifo 
i/o, does not scale linearly to queue depth due to highly non-linear 
variance in callback latencies due to OS caching effects overlaid on 
mechanical motors, so ease for the programmer to fiddle with 
operation chain patterns is paramount for maximum performance, 
especially as it's mainly a trial and error thing given the 
complexities of the systems which make up filing systems etc. One 
basically designs an access pattern you think ought to be performant 
under both cold and warm cache scenarios, try testing it and find 
yourself a bit wide from the mark, so now you hunt around for the 
goldilocks zone through repeated testing cycles. I haven't personally 
found any better way than this yet sadly, storage is so very 
non-linear at the micro-level.

Doing this using ASIO style callbacks involves a ton load of cutting 
and pasting code around, several iterations of compilation to remedy 
the unenforced syntax errors, more iterations of debugging because 
now you've broken the storage etc. Alternatively, doing this using 
AFIO style chained ops is far easier and quicker because you simply 
tweak the op dependency graph you're sending to the async closure 
engine to execute, and you let the engine figure out how best to send 
it to the OS and ASIO.

There is also an additional debugging option made available because 
with a formal op dependency graph one could have code check that your 
sequence of reads and writes is power-loss safe and race condition 
free with other processes reading and writing the same files. Using 
an ASIO style callback model that would be hard without additional 
metadata being supplied.

As much as AFIO style async code is a bit heavy due to the use of 
futures, for the kind of latencies you see with big chunks of data 
being moved around it's an affordable overhead given the convenience.

Niall

-- 
Currently unemployed and looking for work in Ireland.
Work Portfolio: http://careers.stackoverflow.com/nialldouglas/