Diff for "Pradeepkumar"

Google Summer of Code-2010

I am interested in "Parent delta". I can also work on conversion tools. But I am mainly focusing on Parent Delta Plan.

A word about project:

Mercurial calculates diffs against previous revision rather than parent. In some cases this implementation is space inefficient, and it is more sensible to store deltas against parent. This project is about implementing Parent Delta Plan. This implementation changes the structure of revlogs, so wire protocol has to be extended to allow backward compatibility.

Proposal

TO DO(Tentative): Major:

• Changes in revlog
• Changes in wire protocol

Changes in revlog:

• Change revlog structure, now it appends the next delta to existing data. This helps in reconstructing a revision at O(1) seeks. If

parent delta is implemented better compression can be achieved but, may have to compromise with number of seeks.

• Current:Linear delta model

  • * O(1) seeks to create any revision.
• Parent delta model
  • ** I think this can be completed quickly, tonfa shared patches for this***
  • * An intelligent algorithm to reduce number of seeks to reconstruct any revision.I think constructing revision is not straight forward as it is now. It's not going to be linear.Right now constructing a revision is simply applying each block of data to a base till the revision required.
  • * Investigation about merge revision.At merge revision node has two parents, delta should be taken against first or
• * Optimization, and investigation for better implementation(If possible).

Changes in wire protocol:

• Changes in wire protocol to allow backward computability.
• Implementation of changes according to old, new servers, client, repos.
  • *

    Backward compatibility matrix (by tonfa): http://mercurial.selenic.com/wiki/ParentDeltaPlan

• Rigorous testing
• Integrating with other's work.(Changeset discovery, Lightweight copies/renames)

About Me:

• I am student at Indian Institute of Technology, Roorkee. I am currently pursing third year of 5 years degree, Mathematics as major. I am a Linux enthusiast. Interested in open source. I have been learning C, Python. Some of my projects can be found on github ( http://www.github.com/in3Xes ). I have never got opportunity to contribute to open source. GSoC would be stepping stone for that.

• I am project leader in IMG ( Information Management Group, IIT Roorkee, http://www.iitr.ac.in/IMG/ ), a group which manages intranet, student databases, institute website. I have worked on PHP, Java in web development.

Contact Information:

• Name: Pradeepkumar Gayam

  Email: in3xes@gmail.com

• IRC Nick: in3xes (freenode.net)
• Jabber ID: in3xes [AT] gmail.com (talk.google.com)

Schedule:

• I would start coding well before the schedule given in the time line. My final exams will end by May-15th. I can start working rigorously shortly after exams. Till May-15th(or 24th according to time line) I will be reading code, documentation, getting used to coding style, getting ready to start coding, little experimentation etc.

Timeline

Changes in revlog structure: [Total 3 weeks]

• Parent delta model:
  • * An intelligent algorithm to reduce number of seeks to reconstruct any revision.I think constructing revision is not straight forward as it is now. It's not going to be linear.
• [(It can take 1 1/2 to 2 weeks to bring it to working stage, Roughly around 5th-Jun)]
• * Investigation about merge revision.
• * Optimization, and investigation for better implementation(If possible)
• * Continuous parallel testing.
  • [( I would finish testing and other work nearly in 10 days, by 15th-Jun)]

Changes in wire protocol:[Total 3 to 3 1/2 weeks]

• Changes in wire protocol to allow backward computability.
  • [(For midterm evaluation, this may not be complete, but significant part of it)]
  • * Implementation of changes according to old, new servers,client, repos.
    • *

    • Backward compatibility matrix (by tonfa): http://mercurial.selenic.com/wiki/ParentDeltaPlan .

• Rigorous testing.
  • * [(I will start testing shortly after mid-term evaluation, i.e., around 18th-July, would take a week to complete)]
  • * Integrating with other's work. [ ( Remaining time, depending upon other students)]

Link to my GSoC application on bitbucket.org

Work Progress:

Few things related revlog:

1) A script by tonfa, proof for parent delta.

2) mpm explaining how space inefficiency is caused.

3) A python script in mercurial repo to shrink revlog by sorting in topological order.

Earlier in this week I started working on revlog, most of the work is already done by tonfa. He shared patches. All I did was understand what he did, and rewrite those patches with minor changes.

strategy:

As mpm suggested, I have changed the way new revision is being added to existing revlog, and make corresponding changes in creating a given revision of file. The crucial step (performance affecting step) is creating a given version of file. previously to create a given version of a file first we take base revision , to that we keep adding the deltas till given revision. Earlier deltas are added in linear manner. So creating a given revision of file is simply seeking O(1) times.

Defination of base in revlog is not given anywhere. From revlog, a base is the version where we add full text of file to the revlog instead of delta.

example scenario:

We want to create a R1 revision of a file. The parents of R1 are P1 and P2. Base of R1 is B1.

Consider a situation where P2 is null, and P1 < B1. In this case I simply can't take B1 and create R1. I have first find base and parents of P1 and construct P1. The same case might repeat. where parent of P1 is < base of P1. So, first I have to find a base which is ancestor of all the parents, then create a chain of deltas from that base to construct a given revision.

Output of --profile says that, what ever the decrease in the performance is caused only by this function which used for constructing the delta chain. So, I need to find a better algorithm to construct the delta chain. The current algorithm for constructing the delta chain is.

    def deltachain(self, rev):
        chain = []
        while self.base(rev) != rev:
            chain.append(rev)
            rev = self.deltaparent(rev)
        chain.reverse()
        return rev, chain

    def deltaparent(self, rev):
        if self.base(rev) == rev:
            return nullrev
        elif self.flags(rev) & REVLOG_PARENTDELTA_FLAGS:
            return self.parentrevs(rev)[0]
        else:
            return rev-1

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