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SubjectRe: Mercurial 0.4e vs git network pull
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.
> 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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