Date: Thu, 12 May 2005 18:11:49 -0700
From: Matt Mackall <mpm@selenic.com>
To: Daniel Barkalow <barkalow@iabervon.org>
Cc: Petr Baudis <pasky@ucw.cz>, linux-kernel <linux-kernel@vger.kernel.org>,
       git@vger.kernel.org, mercurial@selenic.com,
       Linus Torvalds <torvalds@osdl.org>
Subject: Re: Mercurial 0.4e vs git network pull
Message-ID: <20050513011149.GK5914@waste.org>
References: <20050512222943.GI5914@waste.org> <Pine.LNX.4.21.0505121949210.30848-100000@iabervon.org>
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On Thu, May 12, 2005 at 08:33:56PM -0400, Daniel Barkalow wrote:
> On Thu, 12 May 2005, Matt Mackall wrote:
> 
> > On Thu, May 12, 2005 at 05:24:27PM -0400, Daniel Barkalow wrote:
> > > On Thu, 12 May 2005, Matt Mackall wrote:
> > > 
> > > > Does this need an HTTP request (and round trip) per object? It appears
> > > > to. That's 2200 requests/round trips for my 800 patch benchmark.
> > > 
> > > It requires a request per object, but it should be possible (with
> > > somewhat more complicated code) to overlap them such that it doesn't
> > > require a serial round trip for each. Since the server is sending static
> > > files, the overhead for each should be minimal.
> > 
> > It's not minimal. The size of an HTTP request is often not much
> > different than the size of a compressed file delta.
> 
> I was thinking of server-side processing overhead, not bandwidth. It's
> true that the bandwidth could be noticeable for these small files.
> 
> > All the junk that gets bundled in an http request/response will be
> > similar in size to the stuff in the third column.
> 
> kernel.org seems to send 283-byte responses, to be completely
> precise. This could be cut down substantially if Apache were tweaked a bit
> to skip all the optional headers which are useless or wrong in this
> context. (E.g., that includes sending a content-type of "text/plain" for
> the binary data)
> 
> > Does it do this recursively? Eg, if the server has 800 new linear
> > commits, does the client have to do 800 round trips following parent
> > pointers to find all the new changesets? 
> 
> Yes, although that also includes pulling the commits, and may be
> interleaved with pulling the trees and objects to cover the
> latency. (I.e., one round trip gets the new head hash; the second gets
> that commit; on the third the tree and the parent(s) can be requested at
> once; on the fouth the contents of the tree and the grandparents, at
> which point the bandwidth will probably be the limiting factor for the
> rest of the operation.)

What if a changeset is smaller than the bandwidth-delay product of
your link? As an extreme example, Mercurial is currently at a point
where its -entire repo- changegroup (set of all changesets) can be in
flight on the wire on a typical link.

> > In this case, Mercurial does about 6 round trips, totalling less than
> > 1K, plus one requests that pulls everything.
> 
> I must be misunderstanding your numbers, because 6 HTTP responses is more
> than 1K, ignoring any actual content from the server, and 1K for 800
> commits is less than 2 bytes per commit.

1k of application-level data, sorry. And my whole point is that I
don't send those 800 commit identifiers (which are 40 bytes each as
hex). I send about 30 or so. It's basically a negotiation to find the
earliest commits not known to the client with a minimum of round trips
and data exchange.

> I'm also worried about testing on 800 linear commits, since the projects
> under consideration tend to have very non-linear histories. 

Not true at all. Dumps from Andrew to Linus via patch bombs will
result in runs of hundreds of linear commits on a regular basis.
Linear patch series are the preferred way to make changes and series
of 30 or 40 small patches are not at all uncommon.

-- 
Mathematics is the supreme nostalgia of our time.
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