Hello guys, A few questions/comments/remarks about integer arithmetic and bit operations. And I guess many of my interrogations most likely concern the guys building the VM/primitives... I'm working on a chess engine that will rely heavily on bitboard manipulations, i.e. lots of integer arithmetic with bit operations. I was comparing the 32bit & 64bit Spur VM for Squeak & Pharo (I want the chess engine to be portable on Squeak & Pharo). As I pointed out earlier on on the Squeak-dev mailing list in october 2016, there are a few things that look strange to me and/or that I do not understand. 1) #bitXor: performs the same on the 32bit & 64bit VM for LargePositiveInteger. BUT, on average, #bitXor: is 15 (fifteen) times *slower* on LargePositiveInteger than with SmallInteger, for both 32 & 64 bit VM/Image. Besides, with SmallInteger, #bitXor: is 4.5 faster on 32bit as compared to 64bit!! You can test (on 32bit & 64bit) that with the following snippet: | small1 small2 small3 large1 large2 n timeSmall timeLarge | small1 := 2r1111111111111111. "65535, (1<<16)-1" small2 := (1 << 30) - 1. "Largest SmallInteger on 32bit VM/Image, SmallInteger maxVal" small3 := 1152921504606846975. "Largest Integer on 64bit VM/Image, SmallInteger maxVal" large1 := (1 << 64)-1. "64 bits set to 1" large2 := (1 << 80)-1. "80 bits set to 1" n := 100000000. timeSmall := Time millisecondsToRun: [n timesRepeat: [ small1 bitXor: small2]]. timeLarge := Time millisecondsToRun: [n timesRepeat: [ large1 bitXor: large2]]. Transcript cr;show: 'Time LargePositiveInteger : ', timeLarge printString. Transcript cr;show: 'Time SmallInteger : ', timeSmall printString. As was pointed out by Nicolas Cellier in a private communication, one workaround is to add the method #bitXor: in the class LargePositiveInteger to get a 3x performance boost. Nicolas recommended the following code: LargePositiveInteger>>bitXor: arg <primitive:36> ^super bitXor: arg It's all good and nice (and I do get a 3x speedup, in 32&64 bit) but then I wonder in which respect primitive 36 is different from the one LargePositiveInteger usually inherits from Integer : (Integer>>bitXor:) <primitive: 'primDigitBitXor' module:'LargeIntegers'> Why does Nicolas' workaround is able to do the job (calling primitive 36) when there is already a primitive for the exact thing (<primitive: 'primDigitBitXor' module:'LargeIntegers'>) ? Is this duplicate code or there is a reason for this? And why is the primDigitBitXor primitive so slow as compared to primitive 36? 2) One would normally expect the 64bit VM to be faster than the 32bit VM. That's true in all cases except... Well, first, some numbers (Test case attached to this email if you want to reproduce) 32bit Number of #allMask: per second : 6.20M Number of #anyMask: per second : 7.17M Number of #bitAnd: per second : 8.45M Number of #bitAt: per second : 55.15M Number of #bitAt:put: per second : 37.22M Number of #bitClear: per second : 5.18M Number of #bitInvert per second : 6.53M Number of #bitOr: per second : 9.18M Number of #bitXor: per second : 8.97M Number of #highBit per second : 43.23M Number of #<< per second : 11.34M Number of #lowBit per second : 69.44M Number of #noMask: per second : 7.40M Number of #>> per second : 12.36M 64bit Number of #allMask: per second : 10.26M Number of #anyMask: per second : 10.37M Number of #bitAnd: per second : 17.00M Number of #bitAt: per second : 15.89M Number of #bitAt:put: per second : 10.36M Number of #bitClear: per second : 6.44M Number of #bitInvert per second : 9.11M Number of #bitOr: per second : 18.45M Number of #bitXor: per second : 15.38M Number of #highBit per second : 7.66M Number of #<< per second : 10.50M Number of #lowBit per second : 13.49M Number of #noMask: per second : 11.47M Number of #>> per second : 10.52M There are a few surprises here. First, #bitAt:, #bitAt:put:, #highBit and #lowBit are a *lot faster* on the 32bit VM than on the 64bit VM! Does anyone have an explanation for this? Is this normal? The other surprise is that #>> and #<< seem to be about the same speed on both VM. Any reason why the 64bit version isn't faster than the 32bit one? 3) I was surprised to find out that SmallInteger>>#maxVal wasn't a constant (or the same on both VM). It's (1<<60)-1 on the 64bit VM and (1<<30)-1 on the 32bit VM. So, be warned if your code depends on SmallInteger>>#maxVal for some reason! 4) Is there any rationale/reason/explanation as to why, in Pharo, LargePositiveInteger inherits from the class LargeInteger (which doesn't exist in Squeak) ? Thanks in advance. ----------------- Benoît St-Jean Yahoo! Messenger: bstjean Twitter: @BenLeChialeux Pinterest: benoitstjean Instagram: Chef_Benito IRC: lamneth Blogue: endormitoire.wordpress.com "A standpoint is an intellectual horizon of radius zero". (A. Einstein) TestBitOperations.st (22K) Download Attachment |
> On 06-01-2017, at 11:59 PM, Benoit St-Jean <[hidden email]> wrote: > > 3) I was surprised to find out that SmallInteger>>#maxVal wasn't a constant (or the same on both VM). It's (1<<60)-1 on the 64bit VM and (1<<30)-1 on the 32bit VM. So, be warned if your code depends on SmallInteger>>#maxVal for some reason! > Since the bit size of a SmallInteger is by definition constrained by the word size of the VM, I can’t see why anyone would be surprised by this. SmallInteger is a machine detail specific trick to get basic integer maths to run as fast as we can. One could make a system without it. > > 4) Is there any rationale/reason/explanation as to why, in Pharo, LargePositiveInteger inherits from the class LargeInteger (which doesn't exist in Squeak) ? > A simple matter of factoring preferences. Unless there is a mistake in the refactoring it won’t have any substantial effect. tim -- tim Rowledge; [hidden email]; http://www.rowledge.org/tim Useful random insult:- Teflon brain -- nothing sticks. |
In reply to this post by Benoit St-Jean
Hi Benoit,
On Fri, Jan 6, 2017 at 11:59 PM, Benoit St-Jean <[hidden email]> wrote:
It's really important when running micro-benchmarks like this to minimize or eliminate other costs. So you should unwind your inner loops, to at least to ten repetitions or more. And it's wise to measure the time taken for an empty loop. In these cases I'd consider comparing a loop containing 10 instances of the operation against one containing one, e.g. for a time in seconds you could use | small1 small2 n | n := 1000000. small1 := SmallInteger minVal. small2 := SmallInteger maxVal. (Time millisecondsToRun: [1 to: n do: [:i| small1 bitXor: small2. small1 bitXor: small2. small1 bitXor: small2. small1 bitXor: small2. small1 bitXor: small2. small1 bitXor: small2. small1 bitXor: small2. small1 bitXor: small2. small1 bitXor: small2. small1 bitXor: small2. small1 bitXor: small2]]) - (Time millisecondsToRun: [1 to: n do: [:i| small1 bitXor: small2]]) / 10000.0 / n When I do this I get 2.4e-9 for 64-bit Squeak and 2.7e-9 for 32-bit Squeak. Note that I used 1 to: n do: [:ignored| rather than timesRepeat: because I know its inlined and timesRepeat: isn't always. But in any case I don't see the large difference in SmallInteger operation times that you do. Could you repeat your measurements taking something similar to my approach?
Primitive 36 deals with only 64-bit values (up to 8 byte LargeIntegers). <primitive: 'primDigitBitXor' module:'LargeIntegers'> deals with arbitrary sized large integers.
It depends on the computation. One should expect the 64-bit VM to be faster for certain kinds of arithmetic because it can do more in one operation, e.g. 60-bit arithmetic should be much faster in 64-bits than in 32-bits, most floating point should be much faster because of the immediate floating-point representation. But consider a purely symbolic computation, say tree walking. The 64-bit version has to move twice as much data as the 32-bit version, so if the working set is large but amenable to execution in 32-bits one would expect the 32-bit application to run faster because it accesses half the data.
Can you rerun the numbers using my approach before we attempt to answer this?
_,,,^..^,,,_ best, Eliot |
...and by the way, I got 3.4e-9 on 32-bit (down from 2.7e-9) when I launched it after the 64-bit system and ran the benchmark while the 64-bit system was still running. So run on as quiet a machine as possible. On Sun, Jan 8, 2017 at 4:07 PM, Eliot Miranda <[hidden email]> wrote:
_,,,^..^,,,_ best, Eliot |
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