Re: [JGIT] Questions about binary right shifting

From: Robin Rosenberg <robin.rosenberg.lists@dewire.com>
Date: Feb 5, 2009, 20:32 UTC
Message-ID: <200902052132.01797.robin.rosenberg.lists@dewire.com>
In-Reply-To: <20090205152108.GE26880@spearce.org>

torsdag 05 februari 2009 16:21:08 skrev Shawn O. Pearce:

Show 10 quoted lines

> Yann Simon <yann.simon.fr@gmail.com> wrote:
> > I can see in the code that the signed right shifting is used.
> > Could it be a problem? Or do we manipulate only positive numbers?
> 
> Heh.  We shouldn't ever be dealing with a case where there are more
> than 1 billion files in the index, and thus we should never see
> the expression "low + high" wrap negative, and then try to divide
> by 2 here.
> 
> Binary search on a 1 billion file index would hurt, badly.  I'm not

Roughly three times the search time in the linux kernel repo. Not that bad.

> sure what project even has 1 billion source files to checkout
> on disk.  Isn't that easily a 4 TB filesystem just for the ext2

Not even KDE or Eclipse.

> inodes of the working tree files?  :-)

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> Oh, and we can't even have an index with 1 billion entries in it
> anyway.  DirCache.readFrom allocates a byte[] of entryCnt * 62.
> So our largest number of files permitted is 34,636,833, due to
> the limit on byte[] maxing out at 2 GB.  Thus high below cannot
> ever be larger than 34 million, and we can't wrap.

Assuming the whole index is in memory. But I guess we can discard it as most competing systems choke on a lot lot less and for very large indexes we get other problems with size. Binary search isn't one of them. Having i packed index is probably the biggest issue since it has to be completely rewritten when we add or remove files. But, OK, we won't solve this unless we have a case. I think our current challenge is dealing with 100k files real smoothly.

Patching this to make findbugs shut up is ok with me.

-- robin