Re: Deadlocking in the transport

Dmitri Tikhonov <dtikhonov@litespeedtech.com> Wed, 10 January 2018 22:36 UTC

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Date: Wed, 10 Jan 2018 17:36:42 -0500
From: Dmitri Tikhonov <dtikhonov@litespeedtech.com>
To: Martin Thomson <martin.thomson@gmail.com>
Cc: "Brian Trammell (IETF)" <ietf@trammell.ch>, QUIC WG <quic@ietf.org>
Subject: Re: Deadlocking in the transport
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On Thu, Jan 11, 2018 at 08:54:03AM +1100, Martin Thomson wrote:
> On Wed, Jan 10, 2018 at 6:04 PM, Brian Trammell (IETF) <ietf@trammell.ch>; wrote:
> > Do we have any actual numbers on how much efficiency can actually be
> > squeezed out of allowing compression dictionaries to create unbounded
> > dependencies between streams on common, non-pathological workloads?
> 
> Dmitri ran some tests with QMIN, which doesn't create dependencies,
> and found that for a time compression efficiency degraded noticeably
> if it couldn't create dependencies.  I'll let Dmitri speak to that
> point.  The shortcoming rectifies itself reasonably quickly, though

What we have is a set of simulation results comparing QMIN (conservative:
no blocking) with QCRAM and QPACK (optimistic: blocking allowed).  I used
two web session traces: one visiting www.netbsd.org (small trace) and
another visiting www.facebook.com (larger trace).  I will paste parts of
two reports:

1> I integrated my QMIN library [1] into Alan's proxygen simulator [2] to
1> assess QMIN's compression performance.  Specifically, we want to see how
1> QMIN performs a) in a short-lived connection and b) in a longer connection.
1> I generated two HAR files:
1> 
1>   - I typed in www.netbsd.org into the Location bar of my browser and
1>     waited for the page to load.  This resulted in 18 requests, 17 of
1>     them to www.netbsd.org and 1 to www.paypalobjects.com.
1> 
1>   - I signed in to Facebook and clicked about a dozen links.  This resulted
1>     in 383 requests: about 100 to www.facebook.com, about 280 to fbcdn.net,
1>     and a few odd ones like facebook.net and fbsbx.com.
1> 
1> I left all loss-related simulator options unset for the initial test, as we
1> just want to look at the ideal environment compression performance first.
1> The dynamic table capacity is set to 4 KB.  The results are as follows:
1> 
1> (The simulator implements the latest QPACK and QMIN drafts; its QCRAM
1> implementation dates to June -- but it is better than nothing.)
1> 
1> Table 1: Compression performance comparison, no packet loss
1> 
1>                 HPACK          QPACK          QCRAM          QMIN
1> 
1>   netbsd        1265/6555      1309/6555      1361/6555      2202/6555
1>                 Ratio: 80      Ratio: 80      Ratio: 79      Ratio: 66
1> 
1>   facebook,     58528/243576   61753/243576   61386/243576   63782/243576
1>   blended [a]   Ratio: 75      Ratio: 74      Ratio: 74      Ratio: 73
1> 
1>   facebook,     59325/243576   62594/243576   63462/243576   67492/243576
1>   unblended     Ratio: 75      Ratio: 74      Ratio: 73      Ratio: 72
1> 
1>   a: "blended" means that the simulator pretends that facebook.com and
1>      fbcdn.net is the same domain name.  This is true by default; to
1>      disable, pass -blend=false on the command line.  The unblended
1>      simulation is closer to reality.
1> 
1>   How to read this table:   Each entry is a compressed version of the last
1>   three lines of the simulator output:
1>     Uncompressed Bytes: 243576                67492/243576
1>     Compressed Bytes: 67492         ====>     Ratio: 72
1>     Compression Ratio: 72


The second report addresses compression performance during the first few
flights:

2> What I do have, however, is my short netbsd session (a HAR of which I
2> emailed to the group in an earlier email).  This is as good a starting
2> point as any for drilling down into the compression performance of the
2> first few header blocks.  The netbsd trace contains 17 requests to
2> www.netbsd.org; they belong to three distinct flights:
2> 
2>     1. Request index.html (1 request)
2>     2. Request CSS and JS (3 requests)
2>     3. Request images (13 requests)
2> 
2> It would be informative to examine compression performance in each of
2> the flights.
2> 
2> Since the meeting of Dec 4, Alan added a cumulative compression ratio
2> reporting to the proxygen simulator [2].  In addition, I added a
2> per-header block compression ratio reporting to the QMIN encoder library.
2> First, compression performance comparison:
2> 
2> Table 1: Cumulative compression performance comparison
2> 
2>  Req #      QPACK       QCRAM       QMIN
2>                                    
2>      1      241/39      225/42      303/27
2>                                    
2>      2      78/58       74/60       219/35
2>      3      46/67       44/68       195/39
2>      4      47/72       52/72       188/42
2>                                    
2>      5      52/75       52/75       110/49
2>      6      55/77       55/76       96/53
2>      7      55/78       55/78       96/57
2>      8      54/79       54/78       95/59
2>      9      50/80       50/79       91/61
2>     10      55/80       55/80       96/63
2>     11      56/81       56/80       97/64
2>     12      63/81       63/80       104/65
2>     13      52/81       52/81       93/66
2>     14      51/82       51/81       92/67
2>     15      47/82       47/81       88/67
2>     16      46/82       46/82       87/68
2>     17      79/82       77/81       94/69
2> 
2> How to read this table: the two numbers separated by the slash are the size
2> of the encoded header block followed by the cumulative compression ratio.
2> These values are parsed from the simulator output:
2>     
2>   Encoded request=0 for host=www.netbsd.org block size=225 cumulative \
2>   compression ratio=42
2> 
2>     translates to
2> 
2>   225/42
2> 
2> (Note that the cumulative compression of all three does not match previously
2> reported results: this is because I do not take the last request into account
2> here, as it is going to a different domain.)

Caveat: QCRAM and QPACK drafts have changed significantly since then.
QMIN performance can be improved slightly while still keeping its
non-blocking behavior.

> Roberto makes the point that this is precisely the point where
> compression matters most.  It's a point that the header compression
> folks are still debating.

The opposite view is that packet loss in the beginning would have a worse
impact than compression inefficiency.  I am sorry to say that we do not
have good simulation data to go on.

> (The dependencies wouldn't be unbounded.  One advantage of the
> mechanism that QMIN proposes is that these dependencies are pretty
> narrowly constrained and well understood.)

Yes.  That is, QMIN does not introduce any stream dependencies.

  - Dmitri.