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

[Markus Armbruster]

Daniel P. Berrangé <address@hidden> writes:

> On Fri, Aug 23, 2019 at 07:49:31AM +0200, Markus Armbruster wrote:
>> Daniel P. Berrangé <address@hidden> writes:
>> 
>> > 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.
>> 
>> 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?
>
> I hacked the code to use zlib's crc32 impl and got these for comparison:
>
> /crypto/hash/crc32/speed-512: 1089.18 MB/sec OK
> /crypto/hash/crc32/speed-1024: 1124.63 MB/sec OK
> /crypto/hash/crc32/speed-4096: 1162.73 MB/sec OK
> /crypto/hash/crc32/speed-16384: 1171.58 MB/sec OK
> /crypto/hash/crc32/speed-1048576: 1165.68 MB/sec OK
> /crypto/hash/md5/speed-512: 476.27 MB/sec OK
> /crypto/hash/md5/speed-1024: 517.16 MB/sec OK
> /crypto/hash/md5/speed-4096: 554.70 MB/sec OK
> /crypto/hash/md5/speed-16384: 564.44 MB/sec OK
> /crypto/hash/md5/speed-1048576: 566.78 MB/sec OK

Twice as fast.  Alexey, what do you think?