Re: [boost] GSOC 2015 : Project on Concurrent Hash Tables

Subject: Re: [boost] GSOC 2015 : Project on Concurrent Hash Tables
From: Amarnath V A (me_at_[hidden])
Date: 2015-03-05 19:43:07

On Thu, Mar 5, 2015 at 9:34 PM, Niall Douglas <s_sourceforge_at_[hidden]> wrote:
> On 5 Mar 2015 at 19:34, Amarnath V A wrote:
>
>> I have spent last few days trying to implement the move and copy
>> constructors. But all my trials have ended up in failures.
>
> The fact of failure isn't anything like as important as how you
> failed. Can you tell us some more about exactly what failed and your
> best guesses as to why?

When making changes to move constructor after Rob's and Niall's
review, I ended up several issues. I am not sure if these are due to
my misunderstanding of the data structure or if it's because of my
incorrect implementation. Nevertheless, I am sharing those here.

1. After the discussion with Niall and Rob on the piece of code I have
shared here before, what I understood is that I was not marking the
old map to be reloaded by the threads which are making use of it. Say,
I performed a move construction of old_map and it was being used by
few threads, shouldn't I mark it for reloading by those threads. From
the explanation Niall had provided here, I came to the conclusion that
I have to mark the newly created buckets of old_map as lock state 2.
But this did not work out as expected. I ended up in a infinite loop I
guess when I ran a simple unit test testing the move construction.
Niall, after marking the buckets to be reloaded, how is the actual
reload of the buckets performed? Do I have to some how let the threads
know that you have to do a reload of the buckets? How is this
achieved? I see that the lock stays in state 2 after I set it and is
not switching back to 0 value.

2. For copy construction, should I do similar logic as in _rehash()
method or can I just perform an atomic load and store from the
old_map's buckets? I believe this is not the right way and should do
something similar to the _rehash().

3. How do I handle exceptions in move construction? On an exception,
what is the move construction expected to do? Just abort whatever it
was doing and restore the old_map?

4. And how about copy constructor? Are copy constructors allowed to
throw exceptions?

5. Niall, one  more question. I see that the thread_sanitizer unit
test is not building. Please see the build #228's console output.
https://ci.nedprod.com/view/Boost%20Thread-Expected-Permit/job/Boost.Spinlock%20Test%20Linux%20GCC%204.8/lastCompletedBuild/console

I see that there is an unrecognized flag "-fsanitize=undefined" and
compiler is throwing the following error.

g++: error: unrecognized command line option '-fsanitize=undefined'

I removed the particular flag from the build script and compiled. This
time it worked out and but when I ran, I see so many warnings thrown.
Is this expected or is it that something needs to be fixed?

>
> The only introductory resource I am aware of - and please, if anyone
> else reading can help here please chime in - is Anthony William's
> book "C++ Concurrency in Action: Practical Multithreading". I've
> never read it, but Anthony was one of the main designers of
> Boost.Thread, so I am taking it entirely on trust that his book is
> solid.

Yes, I have already come across this book. I will try to find time and read it.

>
> I had been thinking this topic would be a standard course in Computer
> Science by now, but a quick search of google shows it is an optional
> final year module if present at all in most courses. This is a good
> example of how universities produce students not useful to the
> workplace. That said, MIT appear to have an electable web course on
> shared memory concurrency at
> http://web.mit.edu/6.005/www/fa14/classes/17-concurrency/, this might
> be useful.

I will go through this and see if I am able to do better. Yes, like
you rightly commented, none of the universities have this as even an
elective. What I know about concurrency is just what I have read
online and some from my basic experience writing multi threaded
applications. So, I am at an amateur level.

And I will post any other problems faced from now on. I was under the
impression that as this is part of competency test, the students are
supposed to figure out how to do themselves. Thanks for helping out.

Thanks,
Amarnath