Re: std::deque Thread Saftey Situtation

James Kanze <>
Mon, 1 Sep 2008 13:42:20 -0700 (PDT)
On Sep 1, 6:49 pm, Jerry Coffin <> wrote:

In article <638bd1ab-a5e5-4567-aa53->,

[ ... ]

My advice would be to avoid std::deque in such a situation
-- in a multi-threaded situation, it places an undue
burden on the client code.

Avoid it, or wrap it? I use it regularly for communicating
between threads; my Queue class is based on it.

I'd generally avoid it. Wrapping it works, but presents trade
offs I rarely find useful.

I find the trade off of using tried and tested code, rather than
having to write it myself, useful. (Of course, if you already
had your basic queue working before STL came along...)

The problem with std::deque is that it can and will resize
itself when you add an item and there's not currently room to
store that item. Doing so, however, normally involves
allocating a block of memory from the free store. As such,
this operation can be fairly slow. That translates to a fairly
substantial amount of time that's being spent just in storing
the data rather than processing the data. It only really makes
sense when your work items are sufficiently coarse-grained
that the dynamic allocation will be substantially faster than
processing a single work item.

I suppose that that could be an issue in some cases. For the
most part, my queues never contain very many entries at one
time, although there is a lot of pushing and popping, and the
applications run for very long times (sometimes years without
stopping). Which means that the queue effectively reaches its
largest size very quickly, and then there is no more dynamic
allocation from it. (On the other hand, my messages tend to be
polymorphic, and dynamically allocated.)

I generally prefer to keep the processing sufficiently fine
grained that when/if the queue is full, it's likely to be
faster to wait for an existing item to be processed than to
allocate more memory for the queue. This devotes nearly all
CPU time to processing the data instead of allocating memory
to store it.

I'll admit that I've never had any performance problems with
deque in this regard, although I can easily imagine applications
where it might be the case.

Now, my own work with multiple threads has mostly been fairly
easy to break up into relatively small "chunks", and has been
mostly quite processor intensive. Those mean that 1) giving up
processor time means the consumer side can run faster, and 2)
the time it would take to allocate memory for queue would
exceed the time taken to remove (at least) one item from the

OTOH, I can certainly imagine situations where those weren't
true. An obvious example would be something like a network
router. In this case, the "consumer" side is basically sending
out network packets. More CPU time won't speed up network
transmissions, and transmitting a single network packet
undoubtedly takes longer than allocating a block of memory.

Yes. One of the advantages of forwarding to a queue, and
letting another process handle it, is that you can get back to
listening at the socket that much faster. And since the system
buffers tend to have a fairly small, fixed size...

And if you replace buffer, in_pos and out_pos with
std::deque<T>, where's the problem.

There isn't necessarily a problem -- but I think the choice
between the two is one of some real trade offs rather than
just a matter of whether you manipulate the pointers into the
queue yourself, or use some pre-written code to do the job.

Of course, you can/could also use reserve() and size() on the
deque to manage it as a fixed-size container, but I don't
think at that point you're really gaining much by using a
deque at all. OTOH, it might still make sense when/if you had
both fixed- and variable-sized queues, and this allowed you to
share most code between the two.

In the end, the main gain was that I didn't have to write any
code to manage the queue. And that other people, maintaining
the software, didn't have to understand code that I'd written.

James Kanze (GABI Software)
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