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Opened 18 months ago

Last modified 5 weeks ago

#999 assigned defect

catch limited bandwidth issues sooner

Reported by: Antoine Martin Owned by: Antoine Martin
Priority: major Milestone: 2.1
Component: encodings Version: 0.14.x
Keywords: Cc:


Got some logs which show:

window[2].damage.out_latency.90p : 34
window[2].damage.out_latency.avg : 44
window[2].damage.out_latency.cur : 2
window[2].damage.out_latency.max : 1391
window[2].damage.out_latency.min : 0

Which means that it takes around 44ms to compress and send the packet out to the network layer, often less.
Except that in some cases it can take 1391ms!!

There is another one, which isn't quite as bad:

window[2].damage.out_latency.90p : 324
window[2].damage.out_latency.avg : 119
window[2].damage.out_latency.cur : 25
window[2].damage.out_latency.max : 408
window[2].damage.out_latency.min : 1

At that point the UI became sluggish, about 0.5s behind the actual actions.

Not entirely sure what we should be doing here: by the time the OS is pushing back to us, it is too late already and things will be slow because there isn't enough bandwidth to service us.

Maybe we can watch the "damage out latency" more carefully and immediately increase the batching delay to prevent further degradation?

Change History (11)

comment:1 Changed 17 months ago by Antoine Martin

Status: newassigned

comment:2 Changed 17 months ago by Antoine Martin

Re: linux traffic control via tc.
Just as I remembered it, the documentation is absolutely awful.

First, your need to install kernel-modules-extra: Is Traffic Control (tc) broken in Fedora 17? (Bug 823316 - unable to simulate drops with tc / netem),

The documentation found at the linux foundation is incomplete: http://www.linuxfoundation.org/collaborate/workgroups/networking/netem: when trying to add the latency, you may get this totally unhelpful message: RTNETLINK answers: No such file or directory (which file would that be? no files involved here at all!)
You need to:

modprobe sch_netem

Added difficulty: for testing, it is much easier to run everything on the same system.
Unfortunately, even when using the system's public network IP, the network subsystem will take a shortcut and route through the loopback device.
So you have to apply the rules there, ie:

tc qdisc add dev lo root netem delay 100ms 50ms 25%

And remember to remove them when you're done as this will interfere with lots of things..

tc qdisc del dev lo root netem

For the record, some alternatives:

On OSX and FreeBSD, there's also (obviously) ipfw:

And in OSX Lion onwards with xcode: Network Link Conditioner

Last edited 17 months ago by Antoine Martin (previous) (diff)

comment:3 Changed 16 months ago by Antoine Martin

Milestone: 0.160.17

I have not found a simple solution to this problem - not one that can be merged this late in the release cycle. Re-scheduling. (hopefully some of the changes can be backported).

But I did find a huge bug in the process: r11376. (backported in r11380).

comment:4 Changed 12 months ago by Antoine Martin

  • r12200 (r12202 for v0.16.x branch) should prevent the line jitter from causing drastic changes in the base batch delay
  • r12158 (r12196 for v0.16.x branch) makes it possible to tune the number of soft-expired sends - as this may make things worse on bandwidth constrained links

comment:5 Changed 11 months ago by Antoine Martin

See also #401, #540 and #1135.

Last edited 11 months ago by Antoine Martin (previous) (diff)

comment:6 Changed 11 months ago by alas

Just as a note (as much so I will remember &/or be able to find more easily as for anyone else's benefit), I've managed to get some other tc functions to work as well: loss, reorder, delete all, and list active (examples above for an eth0 device).

  • To list rules: tc -s qdisc ls dev eth0.
  • To add loss: tc qdisc add dev eth0 root netem loss 1%, or tc qdisc add dev eth0 root netem loss 2% 40% to make it more jittery.
  • To add reorder: tc qdisc add dev eth0 root netem reorder 2%, or tc qdisc add dev eth0 root netem reorder 2% 50% to make it more jittery.
  • To delete all the tc rules: tc qdisc del dev eth0 root.

comment:7 Changed 8 months ago by Antoine Martin

The low-level network code is a bit messy, in large part because of win32 and the way it (doesn't) handle blocking sockets...

  • r13270 ensures we don't penalise win32 clients (workaround is now only applied to win32 shadow servers), backported in r13271
  • r13272: code refactoring / cleanup
  • r13273: more detailed packet accounting

At the moment, we detect the network bottleneck because the network write call takes longer to return, handling WSAEWOULDBLOCK and socket.timeout would be more explicit.
Maybe we shouldn't be using blocking sockets? Or maybe reads can be blocking but it would be useful if writes were not so that we could detect when the network layer cannot handle any more data. (assuming that we can distinguish)
Or maybe we need to use different code altogether for win32 and posix?

Related reading:

Last edited 8 months ago by Antoine Martin (previous) (diff)

comment:8 Changed 8 months ago by Antoine Martin

Milestone: 0.171.0

comment:9 Changed 6 months ago by Antoine Martin

Milestone: 1.03.0

Far too late to make intrusive changes to the network layer.
Some recent fixes and breakage: #1211, #1298, #1134

Good read: https://github.com/TigerVNC/tigervnc/wiki/Latency

Last edited 6 months ago by Antoine Martin (previous) (diff)

comment:10 Changed 5 weeks ago by Antoine Martin

Milestone: 3.02.1

comment:11 Changed 5 weeks ago by Antoine Martin

See also #619 and #401

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