Re: [RFC PATCH] Btrfs: add sha256 checksum option

[Alex Elsayed <eternaleye@gmail.com>]

John Williams wrote:

> On Mon, Dec 1, 2014 at 12:08 PM, Alex Elsayed <eternaleye@gmail.com>
> wrote:
>> Actually, I said "Sure" here, but this isn't strictly true. At some
>> point, you're more memory-bound than CPU-bound, and with CPU intrinsic
>> instructions (like SPARC and recent x86 have for SHA) you're often past
>> that. Then, you're not going to see any real difference - and the
>> accelerated cryptographic hashes may even win out, because the intrinsics
>> may be faster (less stuff of the I$, pipelined single instruction beating
>> multiple simpler instructions, etc) than the software non-cryptographic
>> hash.
> 
> In practice, I am skeptical whether any 128- or 256-bit crypto hashes
> will be as fast as the non-crypto hashes I mentioned, even on CPUs
> with specific instructions for the crypto hashes. The non-crypto
> hashes can (and do) take advantage of special CPU instructions as
> well.
> 
> But even if true that the crypto hashes approach the speed of
> non-crypto hashes on certain CPUs, that does not provide a strong
> argument for using the crypto hashes, since on the common x64 CPUs,
> the non-crypto hashes I mentioned are significantly faster than the
> equivalent crypto hashes.
> 
> So, you have some rare architectures where the crypto hashes may
> almost be as fast as the non-crypto, and common CPUs where the
> non-crypto are much faster. That makes the non-crypto hash functions I
> mentioned the obvious choice in the vast majority of systems.

And as I said upthread, one benefit of the Crypto API is that the filesystem 
developers _no longer have to choose_. By using the shash or ahash interface 
to the Crypto API, the _user_ can choose *any* hash the kernel supports. And 
the default is (and will almost certainly continue to be) crc32, so the user 
would need to specify a hash anyway - making whether some other non-
cryptographic hash is the "obvious choice" a completely moot point.