lecture 10 floating point arithmetic gpus in perspective

20
Lecture 10 Floating point arithmetic GPUs in perspective

Upload: trinhdien

Post on 04-Jan-2017

215 views

Category:

Documents


2 download

TRANSCRIPT

Lecture 10

Floating point arithmetic GPUs in perspective

Announcements

•  Interactive use on Forge •  Trestles accounts? A4

©2012 Scott B. Baden /CSE 260/ Winter 2012 2

Today’s lecture

•  Floating point arithmetic •  GPU vs CPUs

©2012 Scott B. Baden /CSE 260/ Winter 2012 3

2/7/12 4 2/7/12 4 2/7/12 4

Floating Point Arithmetic

©2012 Scott B. Baden /CSE 260/ Winter 2012 4

What is floating point? •  A representation

u  ±2.5732… × 1022 u  NaN ∞ u  Single, double, extended precision

•  A set of operations u  + = * / √ rem u  Comparison < ≤ = ≠ ≥> u  Conversions between different formats, binary to decimal u  Exception handling

•  IEEE Floating point standard P754 u  Universally accepted u  W. Kahan received the Turing Award in 1989 for design of IEEE

Floating Point Standard u  Revision in 2008

©2012 Scott B. Baden /CSE 260/ Winter 2012 5

IEEE Floating point standard P754 •  Normalized representation ±1.d…d× 2eps

u  Macheps = Machine epsilon = ε = 2-#significand bits relative error in each operation

u  OV = overflow threshold = largest number u  UN = underflow threshold = smallest number

•  ±Zero: ±significand and exponent = 0

©2012 Scott B. Baden /CSE 260/ Winter 2012

Format # bits #significand bits macheps #exponent bits exponent range ---------- -------- ---------------------- ------------ -------------------- ---------------------- Single 32 23+1 2-24 (~10-7) 8 2-126 - 2127 (~10+-38) Double 64 52+1 2-53 (~10-16) 11 2-1022 - 21023 (~10+-308) Double ≥80 ≥64 ≤2-64(~10-19) ≥15 2-16382 - 216383 (~10+-4932)

Jim Demmel 6

What happens in a floating point operation? •  Round to the nearest representable floating point number

that corresponds to the exact value(correct rounding) •  Round to nearest value with the lowest order bit =0

(rounding toward nearest even) •  Others are possible •  We don’t need the exact value to work this out! •  Applies to + = * / √ rem

•  Error formula: fl(a op b) = (a op b)*(1 + δ) where •  op one of + , - , * , / •  | δ | ≤ ε •  assuming no overflow, underflow, or divide by zero

•  Addition example u  fl(∑ xi) = ∑i=1:n xi*(1+ei) u  |ei| ∼< (n-1)ε

©2012 Scott B. Baden /CSE 260/ Winter 2012 7

Exception Handling •  An exception occurs when the result of a floating point

operation is not representable as a normalized floating point number

u  1/0, √-1

•  P754 standardizes how we handle exceptions u  Overflow: - exact result > OV, too large to represent u  Underflow: exact result nonzero and < UN, too small to represent u  Divide-by-zero: nonzero/0 u  Invalid: 0/0, √-1, log(0), etc. u  Inexact: there was a rounding error (common)

•  Two possible responses u  Stop the program, given an error message u  Tolerate the exeption

©2012 Scott B. Baden /CSE 260/ Winter 2012 8

An example •  Graph the function

f(x) = sin(x) / x

•  But we get a singularity @ x=0: 1/x = ∞ •  This is an “accident” in how we represent the function

(W. Kahan) •  f(0) = 1 •  We catch the exception (divide by 0) •  Substitute the value f(0) = 1

©2012 Scott B. Baden /CSE 260/ Winter 2012 9

Denormalized numbers •  We compute if (a ≠ b) then x = a/(a-b) •  We should never divide by 0, even if a-b is tiny •  Underflow exception occurs when

exact result a-b < underflow threshold UN •  We return a denormalized number for a-b

u  ±0.d…d x 2min_exp

u  sign bit, nonzero significand, minimum exponent value u  Fill in the gap between 0 and UN

©2012 Scott B. Baden /CSE 260/ Winter 2012

Jim Demmel

10

NaN (Not a Number) •  Invalid exception

u  Exact result is not a well-defined real number •  We can have a quiet NaN or an sNan

u  Quiet –does not raise an exception, but propagates a distinguished value

•  E.g. missing data: max(3,NAN) = 3 u  Signaling - generate an exception when accessed

•  Detect uninitialized data

©2012 Scott B. Baden /CSE 260/ Winter 2012 11

Exception handling •  Each of the 5 exceptions manipulates 2 flags •  Sticky flag set by an exception, can be read and cleared by

the user •  Exception flag: should a trap occur?

u  If so, we can enter a trap handler u  But requires precise interrupts, causes problems on a

parallel computer •  We can use exception handling to build faster algorithms

u  Try the faster but “riskier” algorithm u  Rapidly test for accuracy

(possibly with the aid of exception handling) u  Substitute slower more stable algorithm as needed

©2012 Scott B. Baden /CSE 260/ Winter 2012 12

When compiler optimizations alter precision •  Let’s say we support 79+ bit extended format in registers •  When we store values into memory, values a converted to

the lower precision format •  Compilers can keep things in registers and we may lose

referential transparency •  An example

float x, y, z; int j; …. x = y + z; if (x >= j) replace x by something smaller than j // x < j y=x;

•  With optimization turned on, x is computed to extra precision; it is not an ordinary float

•  If x is in a register, x≠y, no guarantee that the condition x < j will be preserved when x is stored in y, i.e. y >= j

©2012 Scott B. Baden /CSE 260/ Winter 2012 13

P754 on the GPU •  Cuda Programming Guide (4.1)

“All compute devices follow the IEEE 754-2008 standard for binary floating-point arithmetic with the following deviations”

u  There is no mechanism for detecting that a floating-point exception has occurred and all operations behave as if the exceptions are always masked… SNaN … are handled as quiet

•  Cap. 2.x: FFMA … is an IEEE-754-2008 compliant fused multiply-add instruction … the full-width product … used in the addition & a single rounding occurs during generation of the final result

u  rnd(A ☓ A + B) with FFMA (2.x) vs rnd(rnd(A ☓ A) + B) FMAD for 1.x

•  FFMA can avoid loss of precision during subtractive cancellation when adding quantities of similar magnitude but opposite signs

•  Also see Precision & Performance: Floating Point and IEEE 754 Compliance for NVIDIA GPUs,” by N. Whitehead and A. Fit-Florea

©2012 Scott B. Baden /CSE 260/ Winter 2012 14

Today’s lecture

•  Floating point arithmetic •  GPU vs CPUs

©2012 Scott B. Baden /CSE 260/ Winter 2012 15

Comparative results with Panfilov Method •  Runs on Forge, single precision, 16 threads on the CPU •  GPU: n= (1024, 1536); t=5.0

(84, 98) GF; Double prec: (49,54) •  CPU: (62,65); Double precision: (35 , 17)

©2012 Scott B. Baden /CSE 260/ Winter 2012 16

Computing Platforms •  NVIDIA GTX-280 – 200 series, like Lilliput •  Intel core i7

u  Superscalar, branch cache, out of order, 2-way hyperthreading u  Each core: 32 KB I+D L1, 256KB L2 u  Shared 8MB L3

©2012 Scott B. Baden /CSE 260/ Winter 2012 17

Workloads (from Lee et al.)

©2012 Scott B. Baden /CSE 260/ Winter 2012 18

Normalized performance

©2012 Scott B. Baden /CSE 260/ Winter 2012 19

Fin