[erlang-bugs] out of memory and crash when using linebuf io

[Bengt Kleberg]

Greetings,

It is a good thing to correct the problem with having no flow control
for line oriented port io.

It would also be (IMHO) a good thing to add the possibility to use
standard io with file:read/2 . For best effect this could be done like
in io:read/3 .


bengt

On Tue, 2008-08-05 at 10:34 +0200, Fredrik Svahn wrote:
> Hi,
> 
> I knew that the new regexp library ("re") in Erlang/OTP R12B-3 is
> fast. But I wanted to know just how fast "erlgrep" could be compared
> to the well-known unix grep which has been around since 1973. My input
> file was quite big (about 980 Mb) and in the first attempt it took
> some 30 minutes for my io:get_line-based program to scan through the
> input compared to 8 minutes for grep. However, I quickly discovered
> that io was the bottleneck rather than the regexps. Also to my
> surprise, the beam process grew until it had a resident size of 950 Mb
> (according to top). Not quite what I had hoped for. I wonder what
> would have happened on my machine with 2 Gb RAM if the file had been
> three times the size?
> 
> I then tried the well-known "open_port({fd,0,1}, [eof,binary,line])"
> approach, and ended up in a program that quickly grew until there was
> no memory left (>1.2 Gb before top stopped working) and then
> *completely* locking up the computer for some 20 minutes before I
> finally managed to take beam down by force.
> 
> I believe the later issue is the same that was discussed a about a
> year ago by Ulf Wiger and others and the suggestion then was to add
> some kind of flow control to line oriented port io. So I hereby humbly
> submit a patch to add some very very basic flow control. I am sure
> that more skilled designers can easily improve it to also handle
> io:get_line() on large files without beam growth. I am also sure there
> is a corner case or two which I have not thought of.
> 
> This is arguably not a bug since piping big chunks of data into beam
> was not what the system originally was built for, but I am still
> hoping to get away with sending it to erlang-bugs and getting it in
> since:
> 
> a) It has been experienced by others as well that it is easy to crash
> beam if you have big piped input files
> b) Currently there is no *fast* way to read data from stdin without
> risking beam growth/crash - this is a serious limitation
> c) I have provided a patch (which also seems to keep the memory
> consumption at more normal levels)
> 
> 
> *** erts/emulator/beam/io.c.old 2008-08-01 00:29:16.000000000 +0200
> --- erts/emulator/beam/io.c     2008-08-04 22:26:00.000000000 +0200
> *************** static void missing_drv_callback(Port *p
> *** 2437,2447 ****
> --- 2437,2472 ----
>   void
>   erts_port_ready_input(Port *p, ErlDrvEvent hndl)
>   {
> +     Process *rp = NULL;
> +     ErtsProcLocks rp_locks = ERTS_PROC_LOCK_MSGQ;
> +
>       ERTS_SMP_CHK_NO_PROC_LOCKS;
>       ERTS_SMP_LC_ASSERT(erts_lc_is_port_locked(p));
> 
>       ASSERT((p->status & ERTS_PORT_SFLGS_DEAD) == 0);
> 
> +     /* When using linebuf_io there is a risk of ending up with an ever growing
> +      * message queue in the connected process since there is no flow control
> +      * and we usually end up with many small messages instead of one large.
> +
> +      * The obvious disadvantage with the simple flow control below is that a
> +      * message queue which has grown for other reasons than io may
> temporarily
> +      * stop io messages to the controlling process. On the other hand if we
> +      * have a message queue of 4*ERL_MESSAGE_BUF_SZ then it is not helping us
> +      * to fill it up with even more messages.
> +
> +      * With this patch at least we will not run out of memory and crash, and
> +      * this makes it easier to analyse the problem... */
> +
> +     if (p->status & ERTS_PORT_SFLG_LINEBUF_IO) {
> +         rp = erts_pid2proc(NULL, 0, p->connected, rp_locks);
> +         if (rp) {
> +            int mq_too_long = rp->msg.len > 4*ERL_MESSAGE_BUF_SZ;
> +            erts_smp_proc_unlock(rp, rp_locks);
> +            if (mq_too_long) return;
> +         }
> +     }
> +
>       if (!p->drv_ptr->ready_input)
>         missing_drv_callback(p, hndl, DO_READ);
>       else {
> 
> 
> BR /Fredrik
> 
> PS. So how did the new re library match up to grep with the above
> patch? Quite well I would say. Almost two minutes faster than grep
> (using the mmap option to grep only made a few seconds difference).
> Even though I knew re was fast from previous experiments I was quite
> pleasantly surprised. It is still using more memory than grep, for
> sure, but still, I am willing to make that trade-off in this case.
> 
> 
> $ time otp_src_R12B-3/bin/erl -smp disable -user logreader <
> /tmp/messages.4 > output2.txt
> 
> real    6m16.410s
> user    6m12.910s
> sys     0m3.220s
> 
> $ time grep -E '(Mar|Apr) *[0-9]+ *0(1|2):' < /tmp/messages.4 > output3.txt
> 
> real    8m17.259s
> user    8m15.170s
> sys     0m1.640s
> 
> $ diff output2.txt output3.txt
> $
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