[Spice-devel] [RFC] Spice channels compression

[Frediano Ziglio]

> 
> Hi,
> 
> Thanks all for your comments
> 
> 
> On 03/03/2017 12:49 PM, Frediano Ziglio wrote:
> >> Hey,
> >>
> >> Do you have any measurements of how much bandwidth is saved on each
> >> channel type on basic usage of linux/windows guests? Ideally, with
> >> numbers about additional CPU usage and added latency?
> >>
> >> Christophe
> >>
> > Yes, without numbers it's a bit hard understand if worst or not.
> 
> The core channels i aimed to were usbredir,webdav and main, i haven't
> came across yet with significant results in other channels.
> The avg latency is about 0.2-0.3 ms per packet when large packet are being
> transferred (tested on usbredir), btw i didn't try anything with windows
> guest yet,
> might be interesting
> 
> Numbers are pretty dependent on what the user is doing, for example:
> ( Number are the compression rate = uncompressed_size/compressed_size)
> 
> 1. usbredir: connecting usb-4GB storage device with ext4 (no files transfer)
> server -> client:
> 2.6
> client -> server:
> 8.2 (4MB)
> 
> 2.webdav : copy compressed JPEG client-> server
> server -> client:
> 1.27
> client -> server:
> 1.7 (copy jpg)
> 

This is quite surprising. jpg should not compress that much (if not at all)
so the 0.7 is expected to came from other data. Seems massive, even
considering that on the reverse direction there's only a 1.27.
This suggests that maybe Christophe hit a good point with its LEB128
suggestion.

> 3. main : It start with impressive compression but afterwards it mostly
> depends on user behavior , for example copy\paste of text should be
> compressed massively in main channel
> server -> client:
> 150.1?! (but just for the first msgs)

Yes, we try to detect the bandwidth sending a bit all zeroes packet :)

> client -> server:
> 2.9
> 
> 4. cursor channel behaves similarly- large compression during boot
> (using client
> mouse after boot) and that's it
> 
> in other words i would describe compression rate for these channels as:
> Usbredir- mostly device dependent, could achieved high compression in
> some common (maybe even the most common) use cases (better than current
> compression)
> webdav\main - it depends what the user is doing\transfer, could achieve
> high compression ratio as well
> >> On Thu, Mar 02, 2017 at 06:53:23PM +0200, Snir Sheriber wrote:
> >>> This series of patches allows compression of messages over
> >>> selected spice channels
> >>>
> > Which channels did you try?
> > Do you have statistics for various channels?
> > I suppose for instance that for sound and display (which are already
> > compressed)
> > is not much worth doing while for instance cursor and usb would gain
> > more (cursor shapes are not currently compressed).
> >
> >>> Few notes:
> >>>
> >>> *Currently lz4 stream and regular compression are in use for small and
> >>> large
> >>> messages accordingly. packets are being sent in common msg type that was
> > Why did you do such distinction for small and large?
> > Have you tried to just use stream all the time?
> 
> Stream compression limitations.. the very large one are pretty rare..
> >>> added,
> >>> and it utilize previous compressed message structure.
> >>>
> >>> *The stream compression & decompression dictionary is based on previous
> >>> messages,
> >>> there for messages that has compressed\decompressed in stream mode are
> >>> being saved
> >>> in continuous pre-allocated buffer and a pointer is used for tracking
> >>> current
> >>> position.
> >>>
> > Wouldn't be worth trying to use framing functions?
> > LZ4 framing functions already handle the buffer for you. They also
> > support flush which would be useful to support large messages (looks
> > like reading the code we are limited to 64K).
> > Would be also possibly worth trying to move the channel compression
> > to lower level (RedsStream) to compress everything and possibly
> > multiple messages together.
> 
> I tried it before on packages that was captured from spice channels
> iirc performance wasn't bad but stream compression was better.. (cpu\
> latency & compression rate)
> I guess it is possible to do something like that (similar idea as the
> tls ?),
> i would say that if it will be implemented in lower level the framing
> lz4 may
> be a wiser choice.
> btw this idea has came up before but i was slightly preferred the
> current one that
> utilize the protocol messages and the already exists compression
> structure. i guess it
> would take some time but i could try it that way also and make measurements
> afterwards..
> 

I did some quick tests with framing, just a small program printing some
information while compressing. I was quite disappointed on the way it works.
Basically it cache the compressed data till you flush or end (which does
a flush) your frame then frame the compressed data and reset the history
(so restarts again). There is a linked flag but still the history is reset.
So basically it does buffering, normal (not streamed) compression and framing.
As flushing is needed to avoid big latency issues resetting the history
kills the compression.

> >>> *Currently all channels are allowed to be compressed, we might want to
> >>> avoid
> >>> compression for some channels (for good or just in specific cases)
> >>> ***please
> >>> notice that usbredir compression was not revert yet so meanwhile i added
> >>> 2
> >>> small
> >>> patches to disable its compression caps.
> >>>
> > Why you had to disable usb ?
> 
> The idea is to revert the usbredir-compression if these patches would be
> upstream eventually, so it's just to avoid double usb compression for now..

Yes, make sense.

> >>> *Multiple clients- basically it should work with more than one client,
> >>> although adding
> >>> compression/decompression to just part of the clients could theoretically
> >>> make it
> >>> worse (good for compression performance testing though)
> >>>
> > It's not clear this point
> 
> ahh, never mind it wont work on the impotent channels anyway
> >>> -If someone has encountered issues with the combination of compression
> >>> and
> >>> other spice features please let me know
> >>> -Suggestions and comments are welcome, especially for improving the
> >>> messages sending  :)
> >>>
> >>> Snir.
> >>>
> >>> Spice components:
> >>> server,client,spice-common,spice-protocol
> >>>
> > Frediano
> Snir.
> 

Wondering if is possible to save the entire stream in a way that is easy
to compute possible different compression framing and schemas.

Frediano