[zeromq-dev] missing messages on 40GbE network

Tue Jul 7 18:33:52 CEST 2015

Futzing about with system tap is what lead us to this. In our case it looks
like the kernel "flush" process is what is holding the lock when we see
this happen.

On Tue, Jul 7, 2015 at 11:22 AM, Marko Vendelin <markov at sysbio.ioc.ee>
wrote:

> The filesystem(s) (31 disks mounted separately) are XFS. I tried ext4 as
> well, no big difference. But failure to acquire lock maybe something to
> look into. System, when stalled, does not have any cpu load and seem to
> idle mainly. How did you find that in your case this function was
> responsible?
>
> Marko
>
> On Tue, Jul 7, 2015 at 6:36 PM, Thomas Rodgers <rodgert at twrodgers.com>
> wrote:
> > Is the filesystem ext4? We have seen issues with high rates of smallish
> > writes to ext4 (it seems related to failing to acquire a lock in
> > http://lxr.free-electrons.com/source/fs/ext4/extents.c?v=2.6.32#L3228).
> >
> > Using XFS seems to improve the situation for us.
> >
> > On Tue, Jul 7, 2015 at 2:16 AM, Marko Vendelin <markov at sysbio.ioc.ee>
> wrote:
> >>
> >> Hi Peter,
> >>
> >> thank you for the pointers. Its seems now that there is some problem
> >> with the disk I/O, as suspected first. Namely, when system starts to
> >> 'crawl', I can fire up new clients that don't write anything and these
> >> clients are doing absolutely fine (recording high rates). New clients
> >> with disk i/o crawl immediately.
> >>
> >> I'll look into it and would try to isolate the issue further.
> >>
> >> REP-REQ: No, I was using multiple requests in PAIR sockets, as you
> >> advised earlier.
> >>
> >> NORM: When things work, TCP is fine. As far as I know a lot is
> >> processed on the cards internally and I can get to the rates that are
> >> as large as needed.
> >>
> >> I'll let the list know if the problem is in disk I/O and what was the
> >> cause of it.
> >>
> >> Best wishes,
> >>
> >> Marko
> >>
> >>
> >> On Mon, Jul 6, 2015 at 11:30 PM, Peter Krey <krey at ripple.com> wrote:
> >> > You may want to try switching to a UDP based protocol like NORM on
> zmq.
> >> > This
> >> > will let you achieve higher throughput as there will be no TCP packet
> >> > handshakes.
> >> >
> >> > You can also try installing multiple NIC cards on your computer and
> bind
> >> > them together into one device for higher throughput if you think the
> >> > cards
> >> > devices buffers are being overrun.
> >> >
> >> > On Mon, Jul 6, 2015 at 1:25 PM, Peter Krey <krey at ripple.com> wrote:
> >> >>
> >> >> You are not using REQ-REP properly; a REQ-REP socket will not accept
> >> >> two
> >> >> REQ messages in a row; it needs a REP before it will proceed
> otherwise
> >> >> it
> >> >> will block.
> >> >>
> >> >> I highly advise you using PAIR type for all sockets in your
> application
> >> >> and no REQ-REP sockets at all, especially given the throughput
> required
> >> >> in
> >> >> your application.
> >> >>
> >> >> On Sun, Jul 5, 2015 at 9:58 AM, Marko Vendelin <markov at sysbio.ioc.ee
> >
> >> >> wrote:
> >> >>>
> >> >>> I did reprogram using pair sockets, one per each client. They were
> >> >>> still
> >> >>> using request reply pattern and when request was not replied to, the
> >> >>> client
> >> >>> repeated the request. Unfortunately, the similar behaviour was
> >> >>> observed:
> >> >>> initial fast rate reduced and never recovered.
> >> >>>
> >> >>> I'm wondering is it possible to get error codes out of zeromq to see
> >> >>> where the problem is?
> >> >>>
> >> >>> Best wishes
> >> >>>
> >> >>> Marko
> >> >>>
> >> >>> On Jul 4, 2015 12:04 AM, "Marko Vendelin" <marko.vendelin at gmail.com
> >
> >> >>> wrote:
> >> >>>>
> >> >>>> Is there any way I could check for automatic drop of messages by
> >> >>>> zeromq? I could recompile the library with some debug settings if
> >> >>>> needed, but this information would be very valuable.
> >> >>>>
> >> >>>> In this case I would expect to have the same in nanomsg as well and
> >> >>>> in
> >> >>>> the beginning of the test with ZeroMQ. We should have disk i/o
> faster
> >> >>>> than the network. Since the dropoff happens at ~10minutes when
> using
> >> >>>> zeromq, RAM would not be able to cache the data either (at that
> time
> >> >>>> I
> >> >>>> have transferred already ~2TB in 64GB RAM machines).
> >> >>>>
> >> >>>> Use of REQ/REP allows me to spread the load among all disks
> >> >>>> automatically. Since disk writers are one per HDD and after
> receiving
> >> >>>> each dataset write it on disk, the load per disk is proportional to
> >> >>>> its speed. The rates I am getting in the beginning with ZMQ (first
> >> >>>> ~10
> >> >>>> min, ~30-36Gb/s) are above our requirements and would fit the
> >> >>>> application perfectly. If I could only sustain it as long as the
> disk
> >> >>>> space allows.
> >> >>>>
> >> >>>> Re PAIR: I was thinking about giving PAIR a try. Would need to
> >> >>>> reprogram a bit, but its possible.
> >> >>>>
> >> >>>> Best wishes,
> >> >>>>
> >> >>>> Marko
> >> >>>>
> >> >>>>
> >> >>>> On Fri, Jul 3, 2015 at 10:52 PM, Peter Krey <peterjkrey at gmail.com>
> >> >>>> wrote:
> >> >>>> > You may be sending messages faster than you can receive them and
> >> >>>> > write
> >> >>>> > them
> >> >>>> > to disk, overflowing zeromq message send buffer causing zeromq to
> >> >>>> > automatically discard some messages. This is expected behavior.
> >> >>>> >
> >> >>>> > Also do not use socket type request reply, use pair. This will
> not
> >> >>>> > require
> >> >>>> > your app to recv and reply before sending the next image; your
> app
> >> >>>> > can
> >> >>>> > send
> >> >>>> > async.
> >> >>>> >
> >> >>>> > On Wednesday, July 1, 2015, Marko Vendelin <markov at sysbio.ioc.ee
> >
> >> >>>> > wrote:
> >> >>>> >>
> >> >>>> >> Dear ØMQ developers:
> >> >>>> >>
> >> >>>> >> Synopsis: I am observing a strange interaction between storing
> >> >>>> >> datastream on harddisks and a loss of ZeroMQ messages. It seems
> >> >>>> >> that
> >> >>>> >> in my use case, when messages are larger than 2MB, some of them
> >> >>>> >> are
> >> >>>> >> randomly dropped.
> >> >>>> >>
> >> >>>> >> Full story:
> >> >>>> >>
> >> >>>> >> I need to pump images acquired by fast scientific cameras into
> the
> >> >>>> >> files with the rates approaching 25Gb/s. For that, images are
> >> >>>> >> acquired
> >> >>>> >> in one server and transferred into the harddisk array using
> 40Gb/s
> >> >>>> >> network. Since Linux-based solutions using iSCSI were not
> working
> >> >>>> >> very
> >> >>>> >> well (maybe need to optimize more) and plain network
> applications
> >> >>>> >> could use the full bandwidth, I decided to use RAID-0 inspired
> >> >>>> >> approach: make filesystem on each of 32 harddisks separately,
> run
> >> >>>> >> small slave programs one per filesystem and let the slaves ask
> the
> >> >>>> >> dataset server for a dataset in a loop. As a messaging system, I
> >> >>>> >> use
> >> >>>> >> ZeroMQ and REQ/REP connection. In general, all seem to work
> >> >>>> >> perfectly:
> >> >>>> >> I am able to stream and record data at about 36Gb/s rates.
> >> >>>> >> However,
> >> >>>> >> at
> >> >>>> >> some point (within 5-10 min), sometimes messages get lost.
> >> >>>> >> Intriguingly, this occurs only if I write files and messages are
> >> >>>> >> 2MB
> >> >>>> >> or larger. Much smaller messages do not seem to trigger this
> >> >>>> >> effect.
> >> >>>> >> If I just stream data and either dump it or just calculate on
> the
> >> >>>> >> basis of it, all messages go through. All messages go through
> if I
> >> >>>> >> use
> >> >>>> >> 1Gb network.
> >> >>>> >>
> >> >>>> >> While in production code I stream data into HDF5, use zmqpp and
> >> >>>> >> pooling to receive messages, I have reduced the problematic code
> >> >>>> >> into
> >> >>>> >> the simplest case using zmq.hpp, regular files, and plain
> >> >>>> >> send/recv
> >> >>>> >> calls. Code is available at
> >> >>>> >>
> >> >>>> >> http://www.ioc.ee/~markov/zmq/problem-missing-messages/
> >> >>>> >>
> >> >>>> >> At the same time, there don't seem to be any excessive drops in
> >> >>>> >> ethernet cards, as reported by ifconfig in Linux (slaves run on
> >> >>>> >> Gentoo, server on Ubuntu):
> >> >>>> >>
> >> >>>> >>
> >> >>>> >> ens1f1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 9000
> >> >>>> >> inet 192.168.38.1 netmask 255.255.255.252 broadcast
> >> >>>> >> 192.168.38.3
> >> >>>> >> inet6 fe80::225:90ff:fe9c:62c3 prefixlen 64 scopeid
> >> >>>> >> 0x20<link>
> >> >>>> >> ether 00:25:90:9c:62:c3 txqueuelen 1000 (Ethernet)
> >> >>>> >> RX packets 8568340799 bytes 76612663159251 (69.6 TiB)
> >> >>>> >> RX errors 7 dropped 0 overruns 0 frame 7
> >> >>>> >> TX packets 1558294820 bytes 93932603947 (87.4 GiB)
> >> >>>> >> TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
> >> >>>> >>
> >> >>>> >> eth3 Link encap:Ethernet HWaddr 00:25:90:9c:63:1a
> >> >>>> >> inet addr:192.168.38.2 Bcast:192.168.38.3
> >> >>>> >> Mask:255.255.255.252
> >> >>>> >> inet6 addr: fe80::225:90ff:fe9c:631a/64 Scope:Link
> >> >>>> >> UP BROADCAST RUNNING MULTICAST MTU:9000 Metric:1
> >> >>>> >> RX packets:1558294810 errors:0 dropped:0 overruns:0
> >> >>>> >> frame:0
> >> >>>> >> TX packets:8570261350 errors:0 dropped:0 overruns:0
> >> >>>> >> carrier:0
> >> >>>> >> collisions:0 txqueuelen:1000
> >> >>>> >> RX bytes:102083292705 (102.0 GB) TX
> >> >>>> >> bytes:76629844394725
> >> >>>> >> (76.6
> >> >>>> >> TB)
> >> >>>> >>
> >> >>>> >>
> >> >>>> >> So, it should not be a simple dropped frames problem.
> >> >>>> >>
> >> >>>> >> Since the problem occurs only with larger messages, is there any
> >> >>>> >> size-limited buffer in ZeroMQ that may cause dropping of the
> >> >>>> >> messages?
> >> >>>> >> Or any other possible solution?
> >> >>>> >>
> >> >>>> >> Thank you for your help,
> >> >>>> >>
> >> >>>> >> Marko
> >> >>>> >> _______________________________________________
> >> >>>> >> zeromq-dev mailing list
> >> >>>> >> zeromq-dev at lists.zeromq.org
> >> >>>> >> http://lists.zeromq.org/mailman/listinfo/zeromq-dev
> >> >>>> >
> >> >>>> >
> >> >>>> > _______________________________________________
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> >> >>>> >
> >> >>>
> >> >>>
> >> >>> _______________________________________________
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> >> >>>
> >> >>
> >> >
> >> >
> >> > _______________________________________________
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