[ kvm-Bugs-2005957 ] TAP networking stalls on large file transfers

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Bugs item #2005957, was opened at 2008-06-29 19:12
Message generated for change (Comment added) made by andyz
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Category: qemu
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Andrew Zabolotny (andyz)
Assigned to: Nobody/Anonymous (nobody)
Summary: TAP networking stalls on large file transfers

Initial Comment:
System: Fedora 9
KVM version: tried 65 and 70, both exhibit the same behaviour

When using TAP networking if I copy a large file from host to guest OS, I get extremely low performance (under 1 megabyte/s) and sometimes file transfer stalls. If I run tcpdump in guest OS (even with a bogus filter - e.g. tcpdump -nn -i eth0 host 1.2.3.4) the stalled connection suddenly come alive. If I run repeatedly tcpdump/press Ctrl+C and so on, the file transfer visually goes much faster.

I found a old thread about the same bug in QEMU: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=290569

The patch proposed there was applied in QEMU 0.7.3, but when I looked into QEMU 0.9.1 sources - it is not there, so it seems it was removed for some reason. However, if I run the same virtual machine with QEMU, I get stable performance with about 8-9 megabytes per second flowing both ways (from host to guest and from guest to host).

I tried to apply the patch on KVM 70, but it does not make any difference :-(

The guest OS is Ubuntu 8.04 but I guess that does not matter.

I will be glad to cooperate to find/fix the bug but for now I'm out of ideas :-(


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>Comment By: Andrew Zabolotny (andyz)
Date: 2008-06-29 20:09

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Update: I was advised on irc to try e1000 network card emulation instead,
and with it I get very good results - 32 megabytes per second and more. So
the bug occurs only with rtl8139 emulation, perhaps because of bandwidth
limitation (packets aren't read quick enough from the TAP handle)?


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