[squeak-dev] [Vm-dev] [Pharo-dev] Byte & String collection hash performance; a modest proposal for change.

Levente Uzonyi leves at caesar.elte.hu
Tue May 2 10:14:12 UTC 2017


On Mon, 1 May 2017, Eliot Miranda wrote:

> 
> 
> On Mon, May 1, 2017 at 5:11 PM, David T. Lewis <lewis at mail.msen.com> wrote:
>       On Mon, May 01, 2017 at 07:26:35PM +0200, Levente Uzonyi wrote:
>       > I presume that a general purpose in-image solution would be more complex.
>       > String already has too many subclasses (6 in Squeak), while at the same
>       > time other kind of new subclasses would be welcome too, e.g. Strings with
>       > 2-byte characters.
>       > Since these properties are orthogonal, there would be many new subclasses
>       > to cover all cases.
>       > Storing the size of the string in a different header word is a VM specific
>       > detail, so I think caching the hash could also be hidden from the image.
>       >
>       > Levente
>
>       Actually, I meant something more like this:
>
>          Object subclass: #LargeString
>               instanceVariableNames: 'anyKindOfString cachedHashValueForTheString'
>               classVariableNames: ''
>               poolDictionaries: ''
>               category: 'Probably a Bad Idea'
> 
>
>       I was guessing that hashing very large strings would imply a somewhat
>       specialized problem domain, for which a wrapper class might make sense.
>       Certainly it would not be a general solution.
> 
> 
> But this implies changing very many places in the VM that access a string as a vector of bytes.  That's why Levente suggests hiding the hash in unused memory in Spur.  It doesn't have the side effect of requiring a major rewrite.

I guess Dave didn't mean the VM to treat LargeString as other Strings.

Levente

> 
>  
>
>       I am probably over my quota of bad ideas for today, so I'll stop now ;-)
>
>       Dave
> 
>
>       >
>       > On Mon, 1 May 2017, David T. Lewis wrote:
>       >
>       > >
>       > >Does it need to be done in the VM? Why not make a class LargeString with
>       > >instance variables aString and myCalculatedHashValueForTheString. That way
>       > >you can cache the hash value and calculate it any way you want.
>       > >
>       > >I know vey little about hashing, just wondering if this kind of thing can
>       > >be handled more easily in the image.
>       > >
>       > >Dave
>       > >
>       > >
>       > >>
>       > >>On Mon, 1 May 2017, Levente Uzonyi wrote:
>       > >>
>       > >>>Well, I had started to write a reply, but I had to postpone it.
>       > >>>I mostly agree with your suggestions.
>       > >>>
>       > >>>One thing that can be done about large strings is to cache the
>       > >>>calculated
>       > >>>hash value in the larger strings. Currently the object representation
>       > >>>changes when the string contains 255 or more characters. In that case an
>       > >>>additional 64 bit field is added to the object header to store its
>       > >>>length.
>       > >>>If we were to use the upper 28+1 bits of that field to cache the hash,
>       > >>>there would still be 35-bits to encode length, which would be enough to
>       > >>>represent strings up to 8 GiB.
>       > >>
>       > >>Well, we can keep the whole range minus one bit, but then we can't store
>       > >>the hash for strings larger than 8 GiB.
>       > >>
>       > >>Levente
>       > >>
>       > >>>But this would require further VM changes (e.g. at:put: would have to
>       > >>>flush the cache).
>       > >>>
>       > >>>Levente
>       > >>>
>       > >>>On Mon, 1 May 2017, Martin McClure wrote:
>       > >>>
>       > >>>>I see no replies to this on any of the three lists it was sent to, so I
>       > >>>>guess I'll chime in.
>       > >>>>
>       > >>>>tl;dr:
>       > >>>>Making a primitive for #hashMultiply, probably a good idea, in some
>       > >>>>form, since doing it in Smalltalk is awkward.
>       > >>>>Only hashing every N-th character of large strings, probably a very bad
>       > >>>>idea. Performance might well get worse, the complexity does not seem
>       > >>>>justified, and it would open a sizeable security hole.
>       > >>>>
>       > >>>>More verbiage below for those interested.
>       > >>>>
>       > >>>>Regards,
>       > >>>>-Martin
>       > >>>>
>       > >>>>On 04/18/2017 07:09 PM, Eliot Miranda wrote:
>       > >>>>>Hi All,
>       > >>>>>
>       > >>>>>    the hash algorithm used for ByteString in Squeak and Pharo is good
>       > >>>>>for "small" strings and overkill for large strings.
>       > >>>>
>       > >>>>Why do you say it's overkill for large strings? Are there applications
>       > >>>>with large strings that are being negatively impacted by the current
>       > >>>>algorithm? Which ones, and impacted how?
>       > >>>>
>       > >>>>>It is important in many applications to get well distributed string
>       > >>>>>hashes, especially over the range of strings that constitute things
>       > >>>>>like method names, URLs, etc.  Consequently, the current algorithm
>       > >>>>>includes every character in a string.  This works very well for
>       > >>>>>"small" strings and results in very slow hashes (and hence long
>       > >>>>>latencies, because the hash is an uninterruptible primitive) for large
>       > >>>>>strings, where large may be several megabytes.
>       > >>>>
>       > >>>>A simple solution for the uninterruptable primitive is to not make it a
>       > >>>>primitive. Make #hashMultiply a primitive (since this particular kind
>       > >>>>of
>       > >>>>numeric modulo computation is really painful in Smalltalk), and do the
>       > >>>>rest in a loop in Smalltalk. It sounds like you've done the
>       > >>>>#hashMultiply primitive already.
>       > >>>>
>       > >>>>If the overhead of calling a primitive for each character proves to be
>       > >>>>too much, even with the faster primitive calling methodologies you
>       > >>>>talked about in the "Cog Primitive Performance" thread on the Vm-dev
>       > >>>>list, a more complex primitive could take a range of bytes, so large
>       > >>>>strings would be done in batches, solving the latency problem.
>       > >>>>
>       > >>>>>
>       > >>>>>Let's look at the basic hash algorithm.
>       > >>>>[...]
>       > >>>>>
>       > >>>>>In looking at this I've added a primitive for hashMultiply; primitive
>       > >>>>>#159 implements precisely self * 1664525 bitAnd: 16r0FFFFFFF for
>       > >>>>>SmallInteger and LargePositiveInteger receivers, as fast as possible
>       > >>>>>in the Cog JIT.  With this machinery in place it's instructive to
>       > >>>>>compare the cost of the primitive against the non-primitive Smalltalk
>       > >>>>>code.
>       > >>>>>
>       > >>>>>First let me introduce a set of replacement hash functions, newHashN.
>       > >>>>>These hash all characters in strings up to a certain size, and then no
>       > >>>>>more than that number for larger strings.  Here are newHash64 and
>       > >>>>>newHash2048, which use pure Smalltalk, including an inlined
>       > >>>>>hashMultiply written to avoid SmallInteger overflow.  Also measured
>       > >>>>>are the obvious variants newHash128, newHash256, newHash512 &
>       > >>>>>mewHash1024.
>       > >>>>>
>       > >>>>>
>       > >>>>[...]
>       > >>>>>So the idea here is to step through the string by 1 for strings sizes
>       > >>>>>up to N - 1, and by greater than 1 for strings of size >= N, limiting
>       > >>>>>the maximum number of characters sampled to between N // 2 and N - 1.
>       > >>>>
>       > >>>>The history of computing is littered with the bones of those who have
>       > >>>>tried this kind of thing. It doesn't end well. Yes, you get a faster
>       > >>>>hash function. And then you find, for sets of data that you or your
>       > >>>>users actually want to use, that you get collisions like crazy, and
>       > >>>>much
>       > >>>>worse overall performance than you started with.
>       > >>>>
>       > >>>>Sure, it works OK for the sets of data that the designer *tested*, and
>       > >>>>probably for the sets of data that they *anticipated*. But real-world
>       > >>>>data is tricky. It includes data sets where the characters that differ
>       > >>>>are the ones that the hash thinks are unimportant, and there goes your
>       > >>>>performance, by orders of magnitude. For instance, early versions of
>       > >>>>Java used a limited number of characters to hash strings. One of the
>       > >>>>biggest compatibility-breaking changes they were forced to make in
>       > >>>>later
>       > >>>>Java versions was to consider *all* characters in hashing. It turned
>       > >>>>out
>       > >>>>that it was very common to hash URLs, and many distinct URLs had most
>       > >>>>of
>       > >>>>their characters in common.
>       > >>>>
>       > >>>>And you don't always get to choose your data -- sometimes you have an
>       > >>>>opponent who is actively looking to create collisions as a
>       > >>>>denial-of-service attack. There was a fair-sized kerfluffle about this
>       > >>>>a
>       > >>>>few years ago -- most web scripting languages made it too easy to mount
>       > >>>>this kind of attack.
>       > >>>>
>       > >>>https://arstechnica.com/business/2011/12/huge-portions-of-web-vulnerable-to-hashing-denial-of-service-attack/
>       > >>>>"...an attacker can degenerate the hash table by sending lots of
>       > >>>>colliding keys. ...making it possible to exhaust hours of CPU time
>       > >>>>using
>       > >>>>a single HTTP request."
>       > >>>>To guard against this kind of attack you need a randomized element in
>       > >>>>your hash (not a bad idea for Smalltalk, actually, and pretty easy --
>       > >>>>mixing in the identity hash of the collection might be sufficient) or a
>       > >>>>cryptographic hash (not worth the computational expense for most
>       > >>>>purposes). However, even adding a randomized element would not prevent
>       > >>>>this kind of attack if you predictably completely ignore some
>       > >>>>characters
>       > >>>>of the input string. That just makes it *so* easy to generate data that
>       > >>>>validates, and is not equal, but causes collisions.
>       > >>>>
>       > >>>>So really, for general-purpose use (i.e. what's built into the
>       > >>>>language)
>       > >>>>hash *every* character of *all* strings. If someone finds that this is
>       > >>>>a
>       > >>>>performance problem in a real-world situation, it can be addressed in
>       > >>>>an
>       > >>>>application-specific way.
>       > >>>
>       > >>>
>       > >>
>       >
> 
> 
> 
> 
> --
> _,,,^..^,,,_
> best, Eliot
> 
>


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