Re: Why do base objects appear behind the delta in packs?

Shawn Pearce <spearce@spearce.org> writes:

>> Shawn Pearce wrote:
>> 
>> > From a data locality perspective putting the base object before
>> > or after the delta shouldn't matter, as either way the delta
>> > is useless without the base.  So placing the base immediately
>> > before the delta should perform just as well as placing it after.
>> > Either way the OS should have the base in cache by the time the
>> > delta is being accessed.
>... 
> I'm going to shutup now and not say anything further on the subject
> unless I've got some hard results indicating a different organization
> is better or worse than what we have right now.

I think that may be a sensible thing to do (no sarcasm -- I
think this measurement is long overdue).

The code was initially proposed just like you suggested but is
in the current form precisely for the reason of avoiding
back-seek.  I distinctly remember me asking Linus "does mmap()
favor forward scan by doing readahead?  I thought its point was
to allow random access" (the answer is "yes" and "yes but
forward is the common case").

The pack-using side in sha1_file.c used to read deltified object
(both header and delta) in full, pick up and read base, and
apply delta to base.  This was thought to be memory hungry on a
longer delta chain, so the current code reads only the header of
a deltified object, reads base, then reads the delta to apply.
The last step involves seeking back, and might make the
back-seek avoidance less effective than before.

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