Re: [Qemu-devel] [RFC PATCH qemu] qapi: Add query-memory-checksum

[Daniel P . Berrangé]

On Thu, Aug 22, 2019 at 04:16:53PM +0200, Markus Armbruster wrote:
> Alexey Kardashevskiy <address@hidden> writes:
> 
> > This returns MD5 checksum of all RAM blocks for migration debugging
> > as this is way faster than saving the entire RAM to a file and checking
> > that.
> >
> > Signed-off-by: Alexey Kardashevskiy <address@hidden>
> 
> Any particular reason for MD5?  Have you measured the other choices
> offered by GLib?
> 
> I understand you don't need crypto-strength here.  Both MD5 and SHA-1
> would be bad choices then.

We have a tests/bench-crypto-hash test but its hardcoded for sha256.
I hacked it to report all algorithms and got these results for varying
input chunk sizes:

/crypto/hash/md5/speed-512: 519.12 MB/sec OK
/crypto/hash/md5/speed-1024: 560.39 MB/sec OK
/crypto/hash/md5/speed-4096: 591.39 MB/sec OK
/crypto/hash/md5/speed-16384: 576.46 MB/sec OK
/crypto/hash/sha1/speed-512: 443.12 MB/sec OK
/crypto/hash/sha1/speed-1024: 518.82 MB/sec OK
/crypto/hash/sha1/speed-4096: 555.60 MB/sec OK
/crypto/hash/sha1/speed-16384: 568.16 MB/sec OK
/crypto/hash/sha224/speed-512: 221.90 MB/sec OK
/crypto/hash/sha224/speed-1024: 239.79 MB/sec OK
/crypto/hash/sha224/speed-4096: 269.37 MB/sec OK
/crypto/hash/sha224/speed-16384: 274.87 MB/sec OK
/crypto/hash/sha256/speed-512: 222.75 MB/sec OK
/crypto/hash/sha256/speed-1024: 253.25 MB/sec OK
/crypto/hash/sha256/speed-4096: 272.80 MB/sec OK
/crypto/hash/sha256/speed-16384: 275.59 MB/sec OK
/crypto/hash/sha384/speed-512: 322.73 MB/sec OK
/crypto/hash/sha384/speed-1024: 369.84 MB/sec OK
/crypto/hash/sha384/speed-4096: 406.71 MB/sec OK
/crypto/hash/sha384/speed-16384: 417.87 MB/sec OK
/crypto/hash/sha512/speed-512: 320.62 MB/sec OK
/crypto/hash/sha512/speed-1024: 361.93 MB/sec OK
/crypto/hash/sha512/speed-4096: 404.91 MB/sec OK
/crypto/hash/sha512/speed-16384: 418.53 MB/sec OK
/crypto/hash/ripemd160/speed-512: 226.45 MB/sec OK
/crypto/hash/ripemd160/speed-1024: 239.25 MB/sec OK
/crypto/hash/ripemd160/speed-4096: 251.31 MB/sec OK
/crypto/hash/ripemd160/speed-16384: 255.01 MB/sec OK


IOW, md5 is clearly the quickest, by a considerable margin over
SHA256/512. SHA1 is slightly slower.

Assuming that we document that this command is intentionally
*not* trying to guarantee collision resistances we're ok.

In fact we should not document what kind of checksum is
reported by query-memory-checksum. The impl should be a black
box from user's POV.

If we're just aiming for debugging tool to detect accidental
corruption, could we even just ignore cryptographic hashs
entirely and do a crc32 - that'd be way faster than even
md5.

Regards,
Daniel
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