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Re: [GSoC][RFC] Proposal: Make pack access code thread-safe




On Mon, Apr 8, 2019 at 3:58 AM Christian Couder
<christian.couder@xxxxxxxxx> wrote:
>
> On Mon, Apr 8, 2019 at 5:32 AM Duy Nguyen <pclouds@xxxxxxxxx> wrote:
> >
> > On Mon, Apr 8, 2019 at 8:23 AM Duy Nguyen <pclouds@xxxxxxxxx> wrote:
> > >
> > > On Mon, Apr 8, 2019 at 5:52 AM Christian Couder
> > > <christian.couder@xxxxxxxxx> wrote:
> > > > > Git has a very optimized mechanism to compactly store
> > > > > objects (blobs, trees, commits, etc.) in packfiles[2]. These files are
> > > > > created by[3]:
> > > > >
> > > > > 1. listing objects;
> > > > > 2. sorting the list with some good heuristics;
> > > > > 3. traversing the list with a sliding window to find similar objects in
> > > > > the window, in order to do delta decomposing;
> > > > > 4. compress the objects with zlib and write them to the packfile.
> > > > >
> > > > > What we are calling pack access code in this document, is the set of
> > > > > functions responsible for retrieving the objects stored at the
> > > > > packfiles. This process consists, roughly speaking, in three parts:
> > > > >
> > > > > 1. Locate and read the blob from packfile, using the index file;
> > > > > 2. If the blob is a delta, locate and read the base object to apply the
> > > > > delta on top of it;
> > > > > 3. Once the full content is read, decompress it (using zlib inflate).
> > > > >
> > > > > Note: There is a delta cache for the second step so that if another
> > > > > delta depends on the same base object, it is already in memory. This
> > > > > cache is global; also, the sliding windows, are global per packfile.
> > > >
> > > > Yeah, but the sliding windows are used only when creating pack files,
> > > > not when reading them, right?
> > >
> > > These windows are actually for reading. We used to just mmap the whole
> > > pack file in the early days but that was impossible for 4+ GB packs on
> > > 32-bit platforms, which was one of the reasons, I think, that sliding
> > > windows were added, to map just the parts we want to read.
> >
> > To clarify (I think I see why you mentioned pack creation now), there
> > are actually two window concepts. core.packedGitWindowSize is about
> > reading pack files. pack.window is for generating pack files. The
> > second window should already be thread-safe since we do all the
> > heuristics to find best base object candidates in threads.
>
> Yeah, it is not very clear in the proposal which windows it is talking
> about as I think a window is first mentioned when describing the steps
> to create a packfile in:
>
> "3. traversing the list with a sliding window to find similar objects
> in the window, in order to do delta decomposing;"
>
> Also the proposal plans to "Protect packfile.c read-and-write global
> variables ..." which made me wonder if it was also about improving
> thread safety when generating pack files.

Sorry, it is indeed unclear. The idea here was to say that variables
which are both read and updated in code that must be thread-safe
should be protected. I will refactor this, thanks.

Oh, also I'm targeting just packfile reading. The explanation on how
packfiles are created was written just as a contextualization. But
perhaps it leaded to some confusion on the proposal's objective.
Thanks for this feedback too.

> Thanks for clarifying!