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Re: Huge win, compressing a window of delta runs as a unit

From
Jon Smirl <jonsmirl@gmail.com>
Date
Aug 17, 2006, 17:17 UTC
Message-ID
<9e4733910608171017k628ef754s4fcd075d60aaf062@mail.gmail.com>
In-Reply-To
<Pine.LNX.4.63.0608171738490.28360@wbgn013.biozentrum.uni-wuerzburg.de>
On 8/17/06, Johannes Schindelin <Johannes.Schindelin@gmx.de> wrote:
Show 18 quoted lines
> Hi,
>
> On Thu, 17 Aug 2006, Jon Smirl wrote:
>
> > On 8/17/06, Johannes Schindelin <Johannes.Schindelin@gmx.de> wrote:
> > > At least, the delta-chains should be limited by size (_not_ by number of
> > > deltas: you can have huge deltas, and if you have to unpack 5 huge deltas
> > > before getting to the huge delta you really need, it takes really long).
> >
> > This is not an obvious conclusion.
>
> The big win is bought by having _one_ zlib stream for multiple deltas,
> right?
>
> So, everytime you want to access the _last_ delta in the chain, you unpack
> _all_ of them. This quite obvious conclusion is obviously your reason to
> propose two packs, one for the archive of "old" objects, and one for the
> "new" objects.

Do some measurements, the IO vs CPU time trade off is way in favor of eliminating the IO. It really doesn't take very much CPU to unpack the delta chain.

The two pack proprosal was not about reducing the delta chain length; they are reverse deltas, the newest version is always at the front. Two packs are used to avoid repacking the 280MB pack when you do a repack command. It takes 2-3 hours to repack 280MB. Even if if you just copy the old pack to the new it take 30 minutes to do all of the IO.

Show 8 quoted lines
> > As for public servers there is an immediate win in shifting to the new
> > pack format.  Three hour downloads vs one hour, plus the bandwidth
> > bills. Are the tools smart enough to say this is a newer pack format,
> > upgrade? It takes far less than two hours to upgrade your git install.
>
> Have you thought about a non-upgraded client? The server has to repack in
> that case, and if the client wants a clone, you might not even have the
> time to kiss your server good-bye before it goes belly up.

Is there a pack format version number built into the server procotol? If not there needs to be one. When there is a mismatch with the server pack version the client should simply display an error requesting an upgrade of the client software.

Git should be designed for forward evolution, not infinite backward compatibility. It is easy to upgrade your client to support the new protocol. The protocol just needs to ensure that the client reliably gets an error about the need to upgrade.

Forward evolution implies that a client is able to work with older servers, but not the inverse, that new servers have to work with old clients.

Show 7 quoted lines
> There is an obvious choice here as to how fast people would upgrade their
> servers.
>
> Ciao,
> Dscho
>
>
-- 
Jon Smirl
jonsmirl@gmail.com
Previous: Nicolas PitreNext: Nicolas Pitre
Message 11 of 36 in “Huge win, compressing a window of delta runs as a unit”
  1. Jon SmirlAug 16, 2006
  2. Shawn PearceAug 17, 2006
  3. Johannes SchindelinAug 17, 2006
  4. Johannes SchindelinAug 17, 2006
  5. Jon SmirlAug 17, 2006
  6. Johannes SchindelinAug 17, 2006
  7. Nicolas PitreAug 17, 2006
  8. Johannes SchindelinAug 17, 2006
  9. Jon SmirlAug 17, 2006
  10. Nicolas PitreAug 17, 2006
  11. Jon SmirlAug 17, 2006
  12. Nicolas PitreAug 17, 2006
  13. Jon SmirlAug 17, 2006
  14. Nicolas PitreAug 17, 2006
  15. Jon SmirlAug 17, 2006
  16. Nicolas PitreAug 17, 2006
  17. Nicolas PitreAug 18, 2006
  18. Jon SmirlAug 18, 2006
  19. Nicolas PitreAug 18, 2006
  20. Jon SmirlAug 18, 2006
  21. Nicolas PitreAug 21, 2006
  22. Shawn PearceAug 21, 2006
  23. Jakub NarebskiAug 21, 2006
  24. Jon SmirlAug 21, 2006
  25. Jon SmirlAug 18, 2006
  26. Johannes SchindelinAug 18, 2006
  27. Jon SmirlAug 18, 2006
  28. Linus TorvaldsAug 19, 2006
  29. Nicolas PitreAug 18, 2006
  30. Shawn PearceAug 21, 2006
  31. Jon SmirlAug 21, 2006
  32. Nicolas PitreAug 21, 2006
  33. Jon SmirlAug 21, 2006
  34. Nicolas PitreAug 21, 2006
  35. Nicolas PitreAug 21, 2006
  36. Nicolas PitreAug 21, 2006

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