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

From: Markus Armbruster <armbru@redhat.com>
To: "Daniel P. Berrangé" <berrange@redhat.com>
Cc: Alexey Kardashevskiy <aik@ozlabs.ru>,
	Paolo Bonzini <pbonzini@redhat.com>,
	qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [RFC PATCH qemu] qapi: Add query-memory-checksum
Date: Fri, 23 Aug 2019 07:49:31 +0200	[thread overview]
Message-ID: <87blwg77o4.fsf@dusky.pond.sub.org> (raw)
In-Reply-To: <20190822144940.GV3267@redhat.com> ("Daniel P. =?utf-8?Q?Berr?= =?utf-8?Q?ang=C3=A9=22's?= message of "Thu, 22 Aug 2019 15:49:40 +0100")

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Thu, Aug 22, 2019 at 04:16:53PM +0200, Markus Armbruster wrote:
>> Alexey Kardashevskiy <aik@ozlabs.ru> 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 <aik@ozlabs.ru>
>> 
>> 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.

Good points.

The doc strings should spell out "for debugging", like the commit
message does, and both should spell out "weak collision resistance".

I can't find CRC-32 in GLib, but zlib appears to provide it:
http://refspecs.linuxbase.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/zlib-crc32-1.html

Care to compare its speed to MD5?