[zeromq-dev] Memory pool for zmq_msg_t

Francesco francesco.montorsi at gmail.com
Tue Aug 13 13:24:10 CEST 2019


Hi all,

today I've taken some time to attempt building a memory-pooling
mechanism in ZMQ local_thr/remote_thr benchmarking utilities.
Here's the result:
 https://github.com/zeromq/libzmq/pull/3631
This PR is a work in progress and is a simple modification to show the
effects of avoiding malloc/free when creating zmq_msg_t with the
standard benchmark utils of ZMQ.

In particular the very fast, zero-lock,
single-producer/single-consumer queue from:
https://github.com/cameron314/readerwriterqueue
is used to maintain between the "remote_thr" main thread and its ZMQ
background IO thread a list of free buffers that can be used.

Here are the graphical results:
with mallocs / no memory pool:
   https://cdn1.imggmi.com/uploads/2019/8/13/9f009b91df394fa945cd2519fd993f50-full.png
with memory pool:
   https://cdn1.imggmi.com/uploads/2019/8/13/f3ae0d6d58e9721b63129c23fe7347a6-full.png

Doing the math the memory pooled approach shows:

mostly the same performances for messages <= 32B
+15% pps/throughput increase @ 64B,
+60% pps/throughput increase @ 128B,
+70% pps/throughput increase @ 210B

[the tests were stopped at 210B because my current quick-dirty memory
pool approach has fixed max msg size of about 210B].

Honestly this is not a huge speedup, even if still interesting.
Indeed with these changes the performances now seem to be bounded by
the "local_thr" side and not by the "remote_thr" anymore. Indeed the
zmq background IO thread for local_thr is the only thread at 100% in
the 2 systems and its "perf top" now shows:

  15,02%  libzmq.so.5.2.3     [.] zmq::metadata_t::add_ref
  14,91%  libzmq.so.5.2.3     [.] zmq::v2_decoder_t::size_ready
   8,94%  libzmq.so.5.2.3     [.] zmq::ypipe_t<zmq::msg_t, 256>::write
   6,97%  libzmq.so.5.2.3     [.] zmq::msg_t::close
   5,48%  libzmq.so.5.2.3     [.]
zmq::decoder_base_t<zmq::v2_decoder_t, zmq::shared_message_memory_allo
   5,40%  libzmq.so.5.2.3     [.] zmq::pipe_t::write
   4,94%  libzmq.so.5.2.3     [.] zmq::shared_message_memory_allocator::inc_ref
   2,59%  libzmq.so.5.2.3     [.] zmq::msg_t::init_external_storage
   1,63%  [kernel]            [k] copy_user_enhanced_fast_string
   1,56%  libzmq.so.5.2.3     [.] zmq::msg_t::data
   1,43%  libzmq.so.5.2.3     [.] zmq::msg_t::init
   1,34%  libzmq.so.5.2.3     [.] zmq::pipe_t::check_write
   1,24%  libzmq.so.5.2.3     [.] zmq::stream_engine_base_t::in_event_internal
   1,24%  libzmq.so.5.2.3     [.] zmq::msg_t::size

Do you know what this stacktrace might mean?
I would expect to have that ZMQ background thread topping in its
read() system call (from TCP socket)...

Thanks,
Francesco


Il giorno ven 19 lug 2019 alle ore 18:15 Francesco
<francesco.montorsi at gmail.com> ha scritto:
>
> Hi Yan,
> Unfortunately I have interrupted my attempts in this area after getting some strange results (possibly due to the fact that I tried in a complex application context... I should probably try hacking a simple zeromq example instead!).
>
> I'm also a bit surprised that nobody has tried and posted online a way to achieve something similar (Memory pool zmq send) ... But anyway It remains in my plans to try that out when I have a bit more spare time...
> If you manage to have some results earlier, I would be eager to know :-)
>
> Francesco
>
>
> Il ven 19 lug 2019, 04:02 Yan, Liming (NSB - CN/Hangzhou) <liming.yan at nokia-sbell.com> ha scritto:
>>
>> Hi,  Francesco
>>    Could you please share the final solution and benchmark result for plan 2?  Big Thanks.
>>    I'm concerning this because I had tried the similar before with zmq_msg_init_data() and zmq_msg_send() but failed because of two issues.  1)  My process is running in background for long time and finally I found it occupies more and more memory, until it exhausted the system memory. It seems there's memory leak with this way.   2) I provided *ffn for deallocation but the memory freed back is much slower than consumer. So finally my own customized pool could also be exhausted. How do you solve this?
>>    I had to turn back to use zmq_send(). I know it has memory copy penalty but it's the easiest and most stable way to send message. I'm still using 0MQ 4.1.x.
>>    Thanks.
>>
>> BR
>> Yan Limin
>>
>> -----Original Message-----
>> From: zeromq-dev [mailto:zeromq-dev-bounces at lists.zeromq.org] On Behalf Of Luca Boccassi
>> Sent: Friday, July 05, 2019 4:58 PM
>> To: ZeroMQ development list <zeromq-dev at lists.zeromq.org>
>> Subject: Re: [zeromq-dev] Memory pool for zmq_msg_t
>>
>> There's no need to change the source for experimenting, you can just use _init_data without a callback and with a callback (yes the first case will leak memory but it's just a test), and measure the difference between the two cases. You can then immediately see if it's worth pursuing further optimisations or not.
>>
>> _external_storage is an implementation detail, and it's non-shared because it's used in the receive case only, as it's used with a reference to the TCP buffer used in the system call for zero-copy receives. Exposing that means that those kind of messages could not be used with pub-sub or radio-dish, as they can't have multiple references without copying them, which means there would be a semantic difference between the different message initialisation APIs, unlike now when the difference is only in who owns the buffer. It would make the API quite messy in my opinion, and be quite confusing as pub/sub is probably the most well known pattern.
>>
>> On Thu, 2019-07-04 at 23:20 +0200, Francesco wrote:
>> > Hi Luca,
>> > thanks for the details. Indeed I understand why the "content_t" needs
>> > to be allocated dynamically: it's just like the control block used by
>> > STL's std::shared_ptr<>.
>> >
>> > And you're right: I'm not sure how much gain there is in removing 100%
>> > of malloc operations from my TX path... still I would be curious to
>> > find it out but right now it seems I need to patch ZMQ source code to
>> > achieve that.
>> >
>> > Anyway I wonder if it could be possible to expose in the public API a
>> > method like "zmq::msg_t::init_external_storage()" that, AFAICS, allows
>> > to create a non-shared zero-copy long message... it appears to be used
>> > only by v2 decoder internally right now...
>> > Is there a specific reason why that's not accessible from the public
>> > API?
>> >
>> > Thanks,
>> > Francesco
>> >
>> >
>> >
>> >
>> >
>> > Il giorno gio 4 lug 2019 alle ore 20:25 Luca Boccassi <
>> > luca.boccassi at gmail.com> ha scritto:
>> > > 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/Publi
>> > > sher.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
>> > > > > >
>> > > > > >
>> > > > >
>> > > > >
>> > > _______________________________________________
>> > > zeromq-dev mailing list
>> > > zeromq-dev at lists.zeromq.org
>> > >
>> > > https://lists.zeromq.org/mailman/listinfo/zeromq-dev
>> > >
>> >
>> >
>> --
>> Kind regards,
>> Luca Boccassi
>> _______________________________________________
>> zeromq-dev mailing list
>> zeromq-dev at lists.zeromq.org
>> https://lists.zeromq.org/mailman/listinfo/zeromq-dev



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