Re: [Boost-users] [thread] multiple producers, single consumer - bosst-specific and generic questions

Subject: Re: [Boost-users] [thread] multiple producers, single consumer - bosst-specific and generic questions
From: Gottlob Frege (gottlobfrege_at_[hidden])
Date: 2009-09-10 16:22:38

On Thu, Sep 10, 2009 at 2:32 PM, John Dlugosz<JDlugosz_at_[hidden]> wrote:

> You could use a ring buffer that the various writers access.  Each does an atomic increment on the W index to obtain a unique index to use, and then populates that element of the buffer's array.  The reader uses the R pointer to consume items.  The trick is, you have several writers all filling their items at the same time, and the R comes along and tries to use one.  The reader needs to know that the writer is done filling it.  So, the reader sets a flag to indicate "not-ready" after it is done using it and before changing R.  The writer populates the fields and then clears the flag, indicating "ready", last.  Now the reader knows to wait if it gets to an element where the flag is still set, even though W has moved ahead.
>

If that particular writing thread stalls (or crashes!?), the reader
thread may have to wait "forever", even though other writes are
finished and waiting.

Not impossible to handle, but something to keep in mind.

Tony

> To preserve the atomicness of the increment-and-read, the W is always incrementing, and never resets.  Instead, the writer has to do the modulo of the ring buffer size when it uses it.
>
> The other hard part is watching for empty or full states.  Decide how to tell them apart, and how to test for them asynchronously and without possibility of a deadlock.
>
> The one I just did like that is specialized, and will never be full.  I just sized the buffer to know how many entries are even possible in my application.
>
> Let me know if you want to discuss it further.
>
> --John
>
>
>
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