Networking: Still waiting for mbuf cluster

Sergei Organov osv at
Fri Oct 24 13:24:21 UTC 2003

Rolf Schroedter <Rolf.Schroedter at> writes:
> My RTEMS application communicates with a Linux-PC via connectionless UDP
> socket, receiving and replying UDP packets every ~20 milliseconds.
> After a few minutes the application stops communicating.
> The application is still running, but no more UDP packets are received.
>  From time to time a message is displayed at the console:
>     "Still waiting for mbuf cluster."
> With [ethereal] I see that correct UDP packets are coming from the PC.
> Additionally the PC is sending ARP requests:
>     "Who has ? Tell"
> which are also not answered by RTEMS.
> What does this mean ?

It means that the stack ran out of mbuf clusters.

> How can the UDP communication be restarted?

It depends on the cause of the problem. Maybe it will help to close the socket
then reopen it. But I'd suggest to fix the cause of the problem rather than
battling with the consequences.

As others mentioned, it could be some mbuf clusters leak in the Ethernet
driver. Not very likely to happen if you use one of the drivers provided with
RTEMS. If you use your own Ethernet driver, make sure there is no mbufs leak
somewhere in the code. A good check for the leak would be to call
'rtems_bsdnet_show_mbuf_stats()' before you open the socket, then let the
socket work for some time, then close it (while it is still operational), then
call the stats routine again and compare the results. If the leak is the case,
the bug needs to be fixed.

It could be the case that you either don't read all the data from the socket
or don't read fast enough and/or write to the socket too fast. In this case
the socket could eat all the mbufs/clusters available to the stack and then
stall. This is the result of general problem with the default stack
configuration in RTEMS, I believe. It seems that the default RTEMS
configuration could be not suitable even for single TCP/UDP socket open by
an application :( At least my recent investigation of similar problem with
TCP sockets lead me to this conclusion.

To understand why the problem could arise and how to eliminate it, let's
consider some numbers. By default, RTEMS allocates 64Kb for mbufs and 128Kb
for clusters for the stack. These defaults are set by 'nmbuf' and
'nmbclusters' variables found in 'libnetworking/rtems/rtems_glue.c'.

[BTW, the next variable defined in this file, 'unsigned long sb_efficiency =
8;' is unused and misleading as its name suggests it has something to do with
the actual 'sb_efficiency' variable that in fact is static in the file

Now, for UDP socket, the stack has receive and transmit buffer size limits
defined by the variables 'udp_recvspace' and 'udp_sendspace' found in the file
'libnetworking/netinet/udp_usrreq.c'. They are 41200 and 9216 bytes,
respectively, so the total configured space for UDP socket is 50416. However,
actual maximum space that the stack may allocate for the socket is higher as
defined by the multiplication factor 'sb_efficiency' found in the file
'libnetworking/kern/uipc_socket2.c'. The default value is 8.

Thus, actual maximum size that the stack could allocate for UDP socket in the
worst case is 50416*8=403328 bytes that is about 4 times more(!) than total
number of bytes available for the stack in clusters (128Kb). Well, the
calculations above are not strict and actual upper margin is somewhat less,
but the estimation we've got at least gives us an idea. Anyway, the BSD stack
starts to behave badly as soon as it runs out of mbufs as allocation of mbufs
is done without timeouts through the stack code. As a result, read/write
timeouts on the socket may stop working and even total unrecoverable stack
stalls are possible. In addition, it seems that once an mbuf/cluster is
allocated to a socket buffer, it won't be returned to the stack until the
socket is closed.

As for TCP sockets, the send/receive buffers are 16Kb each, 32Kb*8=256Kb per
socket maximum that is also greater than total memory allocated for stack by
default. I myself have encountered troubles ("Still waiting for mbufs")
opening three sockets and sending a lot of data in rather small chunks (about
50 bytes each) to them. This results in sockets using no mbuf clusters as such
chunks fit into mbufs. Eventually the result is total stall. It seems that at
some point TCP sockets eat all the mbufs so that there are no spare mbufs to
store incoming packets and pass them to the stack.

Now, what can you do about it? If you have plenty of spare RAM on your board,
just increase the space allocated for mbuf clusters and/or mbufs. Allocating
512Kb for clusters would be definitely enough in your case as above
calculations show.

To decrease memory requirements, you can lower the value of 'sb_efficiency'
and/or 'udp_recvspace'. My problem has gone as soon as I've set sb_efficiency
to 1 and tcp_[send|recv]space to 8Kb while increasing space for mbufs to

Hope it helps.

Sergei Organov.

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