[distcc] small redesign...
Fergus Henderson
fergus at google.com
Sat Nov 1 02:06:12 MDT 2014
Well, perhaps it would be a good idea to add a distccd flag or environment
variable to control the queue length rather than hard-coding 10 or 256?
On 31 Oct 2014 11:37, "Łukasz Tasz" <lukasz at tasz.eu> wrote:
> Hi Guys,
>
> I'm very very happy, reasons of my failures are identified.
> issue is in:
> --- src/srvnet.c (wersja 177)
> +++ src/srvnet.c (kopia robocza)
> @@ -99,7 +99,7 @@
> rs_log_info("listening on %s", sa_buf ? sa_buf : "UNKNOWN");
> free(sa_buf);
>
> - if (listen(fd, 10)) {
> + if (listen(fd, 256)) {
> rs_log_error("listen failed: %s", strerror(errno));
> close(fd);
> return EXIT_BIND_FAILED;
> Index: src/io.c
>
> queue for new connetcion was minited to 10, that's why in case that
> cluster is overloaded, many connection are reseted.
> aim is to even wait 5 min for cluster availability, then compile localy.
>
> @Jarek, thanks for support!
>
> let's discuss if we should fix it or not.
>
> regards
> Lukasz
>
>
> Łukasz Tasz
>
>
> 2014-10-24 10:27 GMT+02:00 Łukasz Tasz <lukasz at tasz.eu>:
> > Hi Martin
> >
> > What I have noticed.
> > Client tries to connect distccd 3 times with 500ms delays in between.
> > Linux kernel by default accept 128 connection.
> > If client creates connection, even if no executors are avaliable,
> > connection is accepted and queued by kernel running distccd.
> > This leads to situation that client thinks that distccd is reserved,
> > but in fact connection still waits to be accepted by distccd server.
> > I suspect that then client starts communication too fast, distcc wont
> > receive DIST token, and both sides waits, communication is broken, and
> > then timeouts are applied for client default is applied, for server
> > there is no defaults.
> >
> > fail scenarion is:
> > one distccd, and two distcc users, both of them will try to compile
> > with DISTCC_HOSTS=distccd/1,cpp,lzo, both users have lot of big
> > objects, cluster is overloaded with ratio 2.
> > This still should be OK, that third, and forth user will join cluster.
> >
> > Easy reproducer is to set one distcc, and set distcc_hosts=distccd/20,
> > this is broken configuration, but simulates overload by 20 - 20
> > developers uses cluster in a same time.
> > Please remember that those are exceptional situation, but developer
> > can start compilation with -j 1000 from his laptop, and cluster will
> > timeout, then receiving 1000 jobs on a laptop will end with memmory
> > killer :D
> > Those are exceptional situation, and somehow cluster should handle that.
> >
> > In the attachement, next to some pump changes, you can find change
> > which is moving making connection to very beginning, when distcc is
> > picking host, also remote connection is made. if this will fail, discc
> > follow default behaviour, goes sleep for one sec, and will pick host
> > again. But this requires additional administration change on distccd
> > machine:
> > iptables -I INPUT -p tcp --dport 3632 -m connlimit --connlimit-above
> > <NUMBER OF DISTCCD> --connlimit-mask 0 -j REJECT --reject-with
> > tcp-reset
> > which accept only number of connection which equals to number of
> executors.
> >
> > So far so good!
> > remark, patch is done on top of arankine_distcc_issue16-r335, since
> > his pump changes are making pump mode working on my environment.
> > But distccd allocation I tested also on latest official distcc release.
> >
> > let me know what you think!
> >
> > with best regards
> > Lukasz
> >
> >
> >
> > Łukasz Tasz
> >
> >
> > 2014-10-24 2:42 GMT+02:00 Martin Pool <mbp at sourcefrog.net>:
> >> It seems like if there's nowhere to execute the job, we want the client
> >> program to just pause, before using too many resources, until it gets
> >> unqueued by a server ready to do the job. (Or, by a local slot being
> >> available.)
> >>
> >>
> >> On Thu Oct 16 2014 at 2:43:35 AM Łukasz Tasz <lukasz at tasz.eu> wrote:
> >>>
> >>> Hi Martin,
> >>>
> >>> Lets assume that you can trigger more compilation tasks executors then
> you
> >>> have.
> >>> In this scenario you are facing situation that cluster is saturated.
> >>> When such a compilation will be triggered by two developers, or two CI
> >>> (e.g jenkins) jobs, then cluster is saturated twice...
> >>>
> >>> Default behaviour is to lock locally slot, and try to connect three
> >>> times, if not, fallback, if fallback is disabled CI got failed build
> >>> (fallback is not the case, since local machine cannot handle -j
> >>> $(distcc -j)).
> >>>
> >>> consider scenario, I have 1000 objects, 500 executors,
> >>> - clean build on one machine takes
> >>> 1000 * 20 sec (one obj) = 20000 / 16 processors = 1000 sec,
> >>> - on cluster (1000/500) * 20 sec = 40 sec
> >>>
> >>> Saturating cluster was impossible without pump mode, but now with pump
> >>> mode after "warm up" effect, pump can dispatch many tasks, and I faced
> >>> situation that saturated cluster destroys almost every compilation.
> >>>
> >>> My expectation is that cluster wont reject my connect, or reject will
> >>> be handled, either by client, either by server.
> >>>
> >>> by server:
> >>> - accept every connetion,
> >>> - fork child if not accepted by child,
> >>> - in case of pump prepare local dir structure, receive headers
> >>> - --critical section starts here-- multi value semaphore with value
> >>> maxchild
> >>> - execute job
> >>> - release semaphore
> >>>
> >>>
> >>> Also what you suggested may be even better solution, since client will
> >>> pick first avaliable executor instead of entering queue, so distcc
> >>> could make connection already in function dcc_lock_one()
> >>>
> >>> I already tried to set DISTCC_DIR on a common nfs share, but in case
> >>> you are triggering so many jobs, this started to be bottle neck... I
> >>> won't tell about locking on nfs, and also scenario that somebody will
> >>> make a lock on nfs and machine will got crash - will not work by
> >>> design :)
> >>>
> >>> I know that scenario is not happening very often, and it has more or
> >>> less picks characteristic, but we should be happy that distcc cluster
> >>> is saturated and this case should be handled.
> >>>
> >>> hope it's more clear now!
> >>> br
> >>> LT
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>> Łukasz Tasz
> >>>
> >>>
> >>> 2014-10-16 1:39 GMT+02:00 Martin Pool <mbp at sourcefrog.net>:
> >>> > Can you try to explain more clearly what difference in queueing
> behavior
> >>> > you
> >>> > expect from this change?
> >>> >
> >>> > I think probably the main change that's needed is for the client to
> ask
> >>> > all
> >>> > masters if they have space, to avoid needing to effectively poll by
> >>> > retrying, or getting stuck waiting for a particular server.
> >>> >
> >>> > On Wed, Oct 15, 2014 at 12:53 PM, Łukasz Tasz <lukasz at tasz.eu>
> wrote:
> >>> >>
> >>> >> Hi Guys,
> >>> >>
> >>> >> please correct me if I'm wrong,
> >>> >> - currently distcc tries to connect server 3 times, with small
> delay,
> >>> >> - server forks x childs and all of them are trying to accept
> incoming
> >>> >> connection.
> >>> >> If server runs out of childs (all of them are busy), client will
> >>> >> fallback, and within next 60 sec will not try this machine.
> >>> >>
> >>> >> What do you think about redesigning distcc in a way that master
> server
> >>> >> will always accept inconing connection, fork a child, but in a same
> >>> >> time only x of them will be able to enter compilation
> >>> >> task(dcc_spawn_child)? (mayby preforking still could be used?)
> >>> >>
> >>> >> This may create kind of queue, client always can decide by his own,
> if
> >>> >> can wait some time, or maximum is DISTCC_IO_TIMEOUT, but still it's
> >>> >> faster to wait, since probably on a cluster side it's just a pick of
> >>> >> saturation then making falback to local machine.
> >>> >>
> >>> >> currently I'm facing situation that many jobs are making fallback,
> and
> >>> >> localmachine is being killed by make's -j calculated for distccd...
> >>> >>
> >>> >> other trick maybe to pick different machine, if current is busy, but
> >>> >> this may be much more complex in my opinion.
> >>> >>
> >>> >> what do you think?
> >>> >> regards
> >>> >> Łukasz Tasz
> >>> >> __
> >>> >> distcc mailing list http://distcc.samba.org/
> >>> >> To unsubscribe or change options:
> >>> >> https://lists.samba.org/mailman/listinfo/distcc
> >>> >
> >>> >
> >>> >
> >>> >
> >>> > --
> >>> > Martin
> __
> distcc mailing list http://distcc.samba.org/
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> https://lists.samba.org/mailman/listinfo/distcc
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