[botan-devel] Threaded Filters/Operation Parallelisation

Joel Low joel at joelsplace.sg
Wed Jan 2 01:51:10 EST 2013

Hello all,

So I've finally managed to sit down and write this patch. The design seems
quite different from what I had in mind, since this round I wanted it to be a
drop-in replacement for Fork.

There's a bit of code taken from a blog post to emulate a semaphore: The link
is there, and if someone knows of a better class (or better still, one within 
Botan itself), let me know and I'll use that class instead.

Let me know what you think.

Patch: http://pastebin.com/kgF0b7Yp (1 month visibility)


-----Original Message-----
From: botan-devel-bounces at randombit.net
[mailto:botan-devel-bounces at randombit.net] On Behalf Of Joel Low
Sent: Tuesday, 24 April 2012 5:07 PM
To: botan-devel at randombit.net
Subject: [botan-devel] Threaded Filters/Operation Parallelisation

Hello all,

Recently I've been playing with the idea of having a threaded Fork filter to
be used together with Pipe: processing for each subsequent filter downstream
of the threaded Fork filter is done in a separate thread of its own. This
could potentially bring performance benefits (in theory) especially with a
move on to having an increasing number of computing cores per CPU.

So I've emailed Jack separate to the list to get some of his opinions. The
main points he raised were that:

 - The approach used for Fork would be most promising in terms of working
with the current design and not forcing a full rewrite of the filter system.
 - He proposed defining a new Filter subclass Threaded_Filter which itself
takes a Filter* as an argument which will spawn a thread and uses two
message queues for I/O with the filter it manages.
 - When write() is called on the Threaded_Filter, it pushes it to the input
queue, which the worker thread pulls off and write()s to the underlying
 - With this approach the application can control concurrency very finely
(perhaps too finely), since the application can specify which filters run on
the main thread, and what runs on a secondary thread.

I was leaning to a slightly different approach: with the knowledge that Fork
operations parallelise the easiest, I should adapt the Fork class to let
each downstream filter run in a separate thread. This shares much
commonality in its design with Jack's idea (especially in terms of thread
sync and friends), provides a little less control over what runs in a
separate thread, but comes with a slightly easier implementation.

Jack stated that I probably should email the devel list and solicit ideas,
since everyone may have different expectations. I'll be implementing this in
my spare time so I'd like to accept as many ideas and combine them before
acting on it. I think that the pipe/filter design Botan has is intuitive,
and I'd like to keep that as much as possible, without compromising on
potential performance gains. Intuitively, both seem to run contrary to each
other, but I think we can work something out here to an API that is both
powerful yet easily mastered.

Any thoughts?


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