[zeromq-dev] Memory pool for zmq_msg_t

[Luca Boccassi]

Another reason for that small struct to be on the heap is so that it
can be shared among all the copies of the message (eg: a pub socket has
N copies of the message on the stack, one for each subscriber). The
struct has an atomic counter in it, so that when all the copies of the
message on the stack have been closed, the userspace buffer
deallocation callback can be invoked. If the atomic counter were on the
stack inlined in the message, this wouldn't work.
So even if room were to be found, a malloc would still be needed.

If you _really_ are worried about it, and testing shows it makes a
difference, then one option could be to pre-allocate a set of these
metadata structures at startup, and just assign them when the message
is created. It's possible, but increases complexity quite a bit, so it
needs to be worth it.

On Thu, 2019-07-04 at 17:42 +0100, Luca Boccassi wrote:
> The second malloc cannot be avoided, but it's tiny and fixed in size
> at
> compile time, so the compiler and glibc will be able to optimize it
> to
> death.
> 
> The reason for that is that there's not enough room in the 64 bytes
> to
> store that structure, and increasing the message allocation on the
> stack past 64 bytes means it will no longer fit in a single cache
> line,
> which will incur in a performance penalty far worse than the small
> malloc (I tested this some time ago). That is of course unless you
> are
> running on s390 or a POWER with 256 bytes cacheline, but given it's
> part of the ABI it would be a bit of a mess for the benefit of very
> few
> users if any.
> 
> So I'd recommend to just go with the second plan, and compare what
> the
> result is when passing a deallocation function vs not passing it (yes
> it will leak the memory but it's just for the test). My bet is that
> the
> difference will not be that large.
> 
> On Thu, 2019-07-04 at 16:30 +0200, Francesco wrote:
> > Hi Stephan, Hi Luca,
> > 
> > thanks for your hints. However I inspected 
> > https://github.com/dasys-lab/capnzero/blob/master/capnzero/src/Publisher.cpp
> > 
> >  and I don't think it's saving from malloc()...  see my point 2)
> > below:
> > 
> > Indeed I realized that probably current ZMQ API does not allow me
> > to
> > achieve the 100% of what I intended to do.
> > Let me rephrase my target: my target is to be able to 
> >  - memory pool creation: do a large memory allocation of, say, 1M
> > zmq_msg_t only at the start of my program; let's say I create all
> > these zmq_msg_t of a size of 2k bytes each (let's assume this is
> > the
> > max size of message possible in my app) 
> >  - during application lifetime: call zmq_msg_send() at anytime
> > always
> > avoiding malloc() operations (just picking the first available
> > unused
> > entry of zmq_msg_t from the memory pool).
> > 
> > Initially I thought that was possible but I think I have identified
> > 2
> > blocking issues:
> > 1) If I try to recycle zmq_msg_t directly: in this case I will fail
> > because I cannot really change only the "size" member of a
> > zmq_msg_t
> > without reallocating it... so that I'm forced (in my example) to
> > always send 2k bytes out (!!)
> > 2) if I do create only a memory pool of buffers of 2k bytes and
> > then
> > wrap the first available buffer inside a zmq_msg_t (allocated on
> > the
> > stack, not in the heap): in this case I need to know when the
> > internals of ZMQ have completed using the zmq_msg_t and thus when I
> > can mark that buffer as available again in my memory pool. However
> > I
> > see that zmq_msg_init_data() ZMQ code contains:
> > 
> >     //  Initialize constant message if there's no need to
> > deallocate
> >     if (ffn_ == NULL) {
> > ...
> >         _u.cmsg.data = data_;
> >         _u.cmsg.size = size_;
> > ...
> >     } else {
> > ...
> >         _u.lmsg.content =
> >           static_cast<content_t *> (malloc (sizeof (content_t)));
> > ...
> >         _u.lmsg.content->data = data_;
> >         _u.lmsg.content->size = size_;
> >         _u.lmsg.content->ffn = ffn_;
> >         _u.lmsg.content->hint = hint_;
> >         new (&_u.lmsg.content->refcnt) zmq::atomic_counter_t ();
> >     }
> > 
> > So that I skip malloc() operation only if I pass ffn_ == NULL. The
> > problem is that if I pass ffn_ == NULL, then I have no way to know
> > when the internals of ZMQ have completed using the zmq_msg_t...
> > 
> > Any way to workaround either issue 1) or issue 2) ?
> > 
> > I understand that the malloc is just of size(content_t)~= 40B...
> > but
> > still I'd like to avoid it...
> > 
> > Thanks!
> > Francesco
> > 
> > 
> > 
> > 
> > 
> > Il giorno gio 4 lug 2019 alle ore 14:58 Stephan Opfer <
> > opfer at vs.uni-kassel.de
> > > ha scritto:
> > > On 04.07.19 14:29, Luca Boccassi wrote:
> > > > How users make use of these primitives is up to them though, I
> > > 
> > > don't
> > > > think anything special was shared before, as far as I remember.
> > > 
> > > Some example can be found here: 
> > > https://github.com/dasys-lab/capnzero/tree/master/capnzero/src
> > > 
> > > 
> > > The classes Publisher and Subscriber should replace the publisher
> > > and 
> > > subscriber in a former Robot-Operating-System-based System. I
> > > hope
> > > that 
> > > the subscriber is actually using the method Luca is talking about
> > > on the 
> > > receiving side.
> > > 
> > > The message data here is a Cap'n Proto container that we
> > > "simply" 
> > > serialize and send via ZeroMQ -> therefore the name Cap'nZero ;-)
> > > 
> > > _______________________________________________
> > > zeromq-dev mailing list
> > > zeromq-dev at lists.zeromq.org
> > > 
> > > 
> > > https://lists.zeromq.org/mailman/listinfo/zeromq-dev
> > > 
> > > 
> > 
> > 
-- 
Kind regards,
Luca Boccassi
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 488 bytes
Desc: This is a digitally signed message part
URL: <https://lists.zeromq.org/pipermail/zeromq-dev/attachments/20190704/bbcd85d4/attachment.sig>