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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.
    >
    > 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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