[mythtv-commits] Ticket #13121: Sat>IP client support

MythTV noreply at mythtv.org
Mon Nov 23 21:00:06 UTC 2020

#13121: Sat>IP client support
 Reporter:  cg@…                |          Owner:  Klaas de Waal
     Type:  Patch - Feature     |         Status:  assigned
 Priority:  minor               |      Milestone:  32.0
Component:  MythTV - Recording  |        Version:  Master Head
 Severity:  low                 |     Resolution:
 Keywords:                      |  Ticket locked:  0

Comment (by Klaas de Waal):

 Testing with the SatIP box the "Sequence errors", other than those that
 appear after tuning to a new channel, are almost all gone when the UDP
 kernel buffers are increased from 256kb, the default on my system, to 4Mb.

 This can be effected with the following commands:

 $ sudo sysctl -w net.core.rmem_max=4194304
 net.core.rmem_max = 4194304
 $ sudo sysctl -w net.core.rmem_default=4194304
 net.core.rmem_default = 4194304


 Investigation shows that the data is not lost in satiprtsp.cpp; all
 packets that are received from the UDPSocket are stored in the buffer and
 retrieved from the buffer without loss.

 There are also no lost packets at the interface level; the network driver
 has no errors and network statistics do not show lost packets.

 Therefore all packets are lost in the UDP buffers; the packets are simply
 not read on time by MythTV. This is demonstrated by the output of

 cat /proc/net/udp

 of which the last column is the number of packets lost.

 Increasing the buffer size does give more latency. A full transport stream
 from a satellite is about 40Mbit/second which is 5Myte/second, so 4Mb
 buffering is 800 milliseconds.

 I do not know how much buffer memory is used by the /dev/dvb/adapter*/*
 drivers but mythtv is capable of reading all data from these devices
 without loss.

 Using TCP instead of UDP will not solve anything as the packets are not
 lost at the network level. TCP allows flow control between MythTV and the
 SatIP box but the SatIP box will be less capable of buffering the stream
 than the MythTV PC is.

 Note that usually the PID filtering is done by the SatIP box; only when
 the number of PIDs is more than 30 the full transport stream is received.
 This is the worst case situation and this can happen with channel scanning
 on some streams.
 Normal recording always uses PID filtering by the SatIP box and then the
 4Mbyte buffer should be big enough for perfect recordings.

Ticket URL: <https://code.mythtv.org/trac/ticket/13121#comment:41>
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