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LCA: How to destroy your community

Posted Jan 24, 2010 17:14 UTC (Sun) by zooko (subscriber, #2589)
In reply to: LCA: How to destroy your community by johnflux
Parent article: LCA: How to destroy your community

Yeah, I was guessing that git is actually network-bound or disk-bound often enough that the CPU hit doesn't matter, but I'm not sure. (According to http://cryptopp.com/benchmarks-amd64.html , SHA-1 is 192 MB/s, SHA-256 is 139 MB/s. Faster than either your disk or your network?)

If git holds out for SHA-3 then hopefully SHA-3 will turn out to be faster than SHA-256. There's even a chance that it will turn out to be faster than SHA-1 on modern CPUs!

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Posted Jan 25, 2010 0:32 UTC (Mon) by johnflux (guest, #58833) [Link]

http://www.mail-archive.com/bug-coreutils@gnu.org/msg1729... That's the thread I was thinking of.

LCA: How to destroy your community

Posted Jan 25, 2010 3:56 UTC (Mon) by njs (guest, #40338) [Link]

SHA-1 can definitely be a bottleneck in some situations. The most extreme -- though I'm not sure whether git does this -- is verifying hashes on an initial clone. (The idea is to prevent one person's disk corruption on an old file or whatever from spreading throughout the network of clones.) Here the disk and network cost is proportional to the size of the delta-compressed repository, but the SHA-1 cost is proportional to the size of the uncompressed repository, which can easily be in the terabyte range.

It can also easily be the bottleneck on, say, committing a large merge (many modified files, all in cache because they were just written).

LCA: How to destroy your community

Posted Jan 27, 2010 11:58 UTC (Wed) by broonie (subscriber, #7078) [Link]

In normal workflows you end up with the files you're accessing in cache so there's no I/O to physical devices and most operations do get CPU bound, especially read only ones. For performance purposes git pretty much assumes that most of the time you're running from hot cache.