1. ME964 High Performance Computing for Engineering Applications
CUDA Arithmetic Support
A Software Design Exercise: Prefix Scan
Oct. 14, 2008

2. Before we get started… Last Time Today
Gauging the extent to which you use hardware resources in CUDA
The occupancy calculator and the “–ptxas-options –v” compile option
Control Flow in CUDA
The issue of divergent warps
Predicated instructions
Today
CUDA arithmetic support
Performance implications
A software design exercise: prefix scan
Preamble to the topic of software design patterns
Helpful with your assignment 2

3. Runtime Math Library There are two types of runtime math operations
__func(): direct mapping to hardware
Fast but low accuracy (see programming guide for details)
Examples: __sin(x), __exp(x), __pow(x,y)
func() : compile to multiple instructions
Slower but higher accuracy (5 ulp, or less)
ulp(x): Units in the Last Place - the gap between the two floating-point numbers closest to the value x
Examples: sin(x), exp(x), pow(x,y)
The -use_fast_math compiler option forces every func() to compile to __func()
Unit in the Last Place or Unit of Least Precision (ULP) is the gap between two very close floating-point numbers. To be exact, ulp(x) is the gap between the two floating-point numbers closest to the value x.
The amount of error in the evaluation of a floating-point operation is often expressed in ULP. An average error of 1 ULP is often seen as a tolerable error.
The error is measured in ulp (units in the last place), that is the values of the results are multiplied by a power of the base such that an error in the last significant place is an error of 1. 3
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4. Arithmetic Instruction Throughput in CUDA
int and float add, shift, min, max and float mul, mad: 4 cycles per warp
int multiply (*) is by default 32-bit
requires multiple cycles / warp
Use __mul24() / __umul24() intrinsics for 4-cycle 24-bit int multiply
Integer divide and modulo are expensive
Compiler will convert literal power-of-2 divides to shifts
Keep this in mind, be explicit in cases where compiler can’t tell that divisor is a power of 2!
Useful trick: foo % n == foo & (n-1) if n is a power of 2 4
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5. Arithmetic Instruction Throughput
Reciprocal, reciprocal square root, sin/cos, log, exp: 16 cycles per warp
These are the versions prefixed with “__”
Examples:__rcp(), __sin(), __exp()
Other functions are combinations of the above
y / x == rcp(x) * y == 20 cycles per warp
sqrt(x) == rcp(rsqrt(x)) == 32 cycles per warp 5
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6. Make your program float-safe!
Existing hardware already has double precision support
G80 is single-precision only
Double precision comes at a performance cost
Careless use of double or undeclared types may run more slowly on existing cards with double precision support (GTX280)
Important to be float-safe (be explicit whenever you want single precision) to avoid using double precision where it is not needed
Add ‘f’ specifier on float literals:
foo = bar * 0.123; // double assumed
foo = bar * 0.123f; // float explicit
Use float version of standard library functions
foo = sin(bar); // double assumed
foo = sinf(bar); // single precision explicit 6
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7. Deviations from IEEE-754
Addition and Multiplication are IEEE 754 compliant
Maximum 0.5 ulp (units in the least place) error
However, often combined into multiply-add (FMAD)
Intermediate result is truncated
Division is non-compliant (2 ulp)
Not all rounding modes are supported
No mechanism to detect floating-point exceptions
The error is measured in ulp (units in the last place), that is the values of the results are multiplied by a power of the base such that an error in the last significant place is an error of 1. 7
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8. GPU Floating Point Features
SSE
IBM Altivec
Cell SPE
Precision
IEEE 754
Rounding modes for FADD and FMUL
Round to nearest and round to zero
All 4 IEEE, round to nearest, zero, inf, -inf
Round to nearest only
Round to zero/truncate only
Denormal handling
Flush to zero
Supported, 1000’s of cycles
NaN support
Yes No
Overflow and Infinity support
Yes, only clamps to max norm
No, infinity
Flags
Some
Square root
Software only
Hardware
Division
Reciprocal estimate accuracy
24 bit
12 bit
Reciprocal sqrt estimate accuracy
23 bit
log2(x) and 2^x estimates accuracy
Streaming SIMD Extensions (SSE) is a SIMD (Single Instruction, Multiple Data) instruction set extension to the x86 architecture, designed by Intel and introduced in 1999 in their Pentium III series processors as a reply to AMD's 3DNow! (which had debuted a year earlier). SSE contains 70 new instructions. 8
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9. End: CUDA Arithmetic Support
Begin: Software Design Exercise ~Prefix Sum~
End 9

10. Objective Putting your CUDA knowledge to work
The vehicle for the software design exercise today is the parallel implementation of a prefix sum operation
Recall first assignment, also the topic of the current assignment
Understand that:
Different algorithmic designs lead to different performance levels
Different constraints dominate in different applications and/or design solutions
Case studies help to establish intuition, idioms and ideas
Point out parallel algorithm patterns that can result in superior performance
Understand that there are patterns and it’s worth being aware of them
If you want, these are the tricks of the trade
When considering patterns, you can’t lose sight of the underlying hardware 10

11. Software for Parallel Computers
You come to rely on compiler to figure out the parallelism in a piece of code and then map it to an underlying hardware
VERY hard, the holy grail in parallel computing
You rely on parallel libraries built for a specific underlying hardware
Very convenient, the way to go when such libraries are available
You rely on language extensions to facilitate the process of generating a parallel executable
This is where you are with CUDA 11

12. Parallel Prefix Sum (Scan)
Definition:
The all-prefix-sums operation takes a binary associative operator  with identity I, and an array of n elements
[a0, a1, …, an-1]
and returns the ordered set
[I, a0, (a0  a1), …, (a0  a1  …  an-2)].
Example: if  is addition, then scan on the set
[ ]
returns the set
[ ]
Exclusive scan: last input element is not included in the result
(From Blelloch, 1990, “Prefix Sums and Their Applications) 12

13. Applications of Scan
Scan is a simple and useful parallel building block
Convert recurrences from sequential … for(j=1;j<n;j++) out[j] = out[j-1] + f(j);
… into parallel:
forall(j) in parallel
temp[j] = f(j); scan(out, temp);
Useful in implementation of several parallel algorithms:
radix sort
quicksort
String comparison
Lexical analysis
Stream compaction
Polynomial evaluation
Solving recurrences
Tree operations
Histograms
Etc. 13
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14. Scan on the CPU
void scan( float* scanned, float* input, int length) {
scanned[0] = 0;
for(int i = 1; i < length; ++i)
scanned[i] = scanned[i-1] + input[i-1]; }
Just add each element to the sum of the elements before it
Trivial, but sequential
Exactly n-1 adds: optimal in terms of work efficiency
(Dan): n-1 instead of n adds 14

15. Parallel Scan Algorithm: Solution One Hillis & Steele (1986)
Note that a implementation of the algorithm shown in picture requires two buffers of length n (shown is the case n=8=23)
Assumption: the number n of elements is a power of 2: n=2M 15
Picture courtesy of Mark Harris

16. The Plain English Perspective
First iteration, I go with stride 1=20
Start at x[2M] and apply this stride to all the array elements before x[2M] to find the mate of each of them. When looking for the mate, the stride should not land you before the beginning of the array. The sum replaces the element of higher index.
This means that I have 2M-1 additions
Second iteration, I go with stride 2=21
This means that I have 2M – 21 additions
Third iteration: I go with stride 4=22
This means that I have 2M – 22 additions
… (and so on) 16

17. The Plain English Perspective
Consider the kth iteration (k is some arbitrary valid integer): I go with stride 2k-1
Start at x[2M] and apply this stride to all the array elements before x[2M] to find the mate of each of them. When looking for the mate, the stride should not land you before the beginning of the array. The sum replaces the element of higher index.
This means that I have 2M-2k-1 additions …
Mth iteration: I go with stride 2M-1
This means that I have 2M-2M-1 additions
NOTE: There is no (M+1)th iteration since this would automatically put me beyond the bounds of the array (if you apply an offset of 2M to “&x[2M] ” it places you right before the beginning of the array – not good…) 17

18. Hillis & Steele Parallel Scan Algorithm
Algorithm looks like this:
for d := 0 to M-1 do
forall k in parallel do
if k – 2d ≥0 then
x[out][k] := x[in][k] + x[in][k − 2d]
else
x[out][k] := x[in][k]
endforall
swap(in,out)
endfor
Double-buffered version of the sum scan 18

19. Operation Count Final Considerations
The number of operations tally:
(2M-20) + (2M-21) + … + (2M-2k) +…+ (2M-2M-1)
Final operation count:
This is an algorithm with O(n*log(n)) work
This scan algorithm is not that work efficient
Sequential scan algorithm does n-1 adds
A factor of log(n) might hurt: 20x more work for 106 elements!
Homework requires a scan of about 16 million elements
A parallel algorithm can be slow when execution resources are saturated due to low algorithm efficiency 19

20. Hillis & Steele: Kernel Function
__global__ void scan(float *g_odata, float *g_idata, int n) {
extern __shared__ float temp[]; // allocated on invocation
int thid = threadIdx.x;
int pout = 0, pin = 1;
// load input into shared memory.
// Exclusive scan: shift right by one and set first element to 0
temp[thid] = (thid > 0) ? g_idata[thid-1] : 0;
__syncthreads();
for( int offset = 1; offset < n; offset <<= 1 )
pout = 1 - pout; // swap double buffer indices
pin = 1 - pout;
if (thid >= offset)
temp[pout*n+thid] += temp[pin*n+thid - offset];
else
temp[pout*n+thid] = temp[pin*n+thid]; }
g_odata[thid] = temp[pout*n+thid1]; // write output 20

21. Hillis & Steele: Kernel Function, Quick Remarks
The kernel is very simple, which is good
Note the nice trick that was used to swap the buffers
The kernel only works when the entire array is processed by one block
One block in CUDA has 512 threads, which means I can have up to 1024 elements (short of 16 million, which is your assignment)
This needs to be improved upon, can’t limit solution to what’s been presented so far 21

22. Improving Efficiency A common parallel algorithm pattern:
Balanced Trees
Build a balanced binary tree on the input data and sweep it to and then from the root
Tree is not an actual data structure, but a concept to determine what each thread does at each step
For scan:
Traverse down from leaves to root building partial sums at internal nodes in the tree
Root holds sum of all leaves (this is a reduction algorithm!)
Traverse back up the tree building the scan from the partial sums 22
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23. Picture and Pseudocode ~ Reduction Step~
i2 =
NOTE: “-1” entries
indicate no-ops
for k=0 to M-1
offset = 2k
for j=1 to 2M-k-1 in parallel do
x[j·2k+1-1] = x[j·2k+1-1] + x[j·2k+1-2k-1]
endfor 23

24. Validate the Indices/Offset
function [offset,index1,index2] = coeffsReduce(M)
% MATLAB utility to validate my understanding of
% how the various strides and indeces are to be
% constructed in the up-sweep phase of the
% Blelloch scan algorithm
offset = zeros(M,1);
offset = offset - 1;
index1 = zeros(M, 2^(M-1));
index1 = index1 - 1;
index2 = zeros(M, 2^(M-1));
index2 = index2 - 1;
for k=0:M-1
offset(k+1) = 2^k;
for j=1:2^(M-k-1)
index1(k+1,j) = j*2^(k+1)-1;
index2(k+1,j) = j*2^(k+1)-1-2^k;
end
Validate the Indices/Offset
>> [offset, i1, i2] = coeffsReduce(3)
offset = 1 2 4
i1 =
i2 = 24

25. Operation Count, Reduce Phase
for k=0 to M-1
offset = 2k
for j=1 to 2M-k-1 in parallel do
x[j·2k+1-1] = x[j·2k+1-1] + x[j·2k+1-2k-1]
endfor
By inspection:
Looks promising… 25

26. The Down-Sweep Phase NOTE: This is just a mirror
image of the reduction
stage. Easy to come up with
the indexing scheme…
for k=M-1 to 0
offset = 2k
for j=1 to 2M-k-1 in parallel do
dummy = x[j·2k+1-2k-1]
x[j·2k+1-2k-1] = x[j·2k+1-1]
x[j·2k+1-1] = x[j·2k+1-1] + dummy
endfor 26

27. Down-Sweep Phase, Remarks
Number of operations for the down-sweep phase:
Additions: n-1
Swaps: n-1 (each swap shadows an addition)
Total number of operations associated with this algorithm
Additions: 2n-2
Swaps: n-1
Looks very comparable with the work load in the sequential solution
The algorithm is convoluted though, it won’t be easy to implement
Kernel shown on next slide 27

28. 28 01| __global__ void prescan(float *g_odata, float *g_idata, int n)
02| {
03| extern __shared__ float temp[];// allocated on invocation
04|
05|
06| int thid = threadIdx.x;
07| int offset = 1;
08|
09| temp[2*thid] = g_idata[2*thid]; // load input into shared memory
10| temp[2*thid+1] = g_idata[2*thid+1];
11|
12| for (int d = n>>1; d > 0; d >>= 1) // build sum in place up the tree
13| {
14| __syncthreads();
15|
16| if (thid < d)
17| {
18| int ai = offset*(2*thid+1)-1;
19| int bi = offset*(2*thid+2)-1;
20|
21| temp[bi] += temp[ai];
22| }
23| offset *= 2;
24| }
25|
26| if (thid == 0) { temp[n - 1] = 0; } // clear the last element
27|
28| for (int d = 1; d < n; d *= 2) // traverse down tree & build scan
29| {
30| offset >>= 1;
31| __syncthreads();
32|
33| if (thid < d)
34| {
35| int ai = offset*(2*thid+1)-1;
36| int bi = offset*(2*thid+2)-1;
37|
38| float t = temp[ai];
39| temp[ai] = temp[bi];
40| temp[bi] += t;
41| }
42| }
43|
44| __syncthreads();
45|
46| g_odata[2*thid] = temp[2*thid]; // write results to device memory
47| g_odata[2*thid+1] = temp[2*thid+1];
48| } 28

29. Bank Conflicts Current implementation has many ShMem bank conflicts
Can significantly hurt performance on current GPU hardware
The source of the conflicts: linear indexing with stride that is a power of 2 multiple of thread id (see below): “j·2k+1-1”
for k=0 to M-1
offset = 2k
for j=1 to 2M-k-1 in parallel do
x[j·2k+1-1] = x[j·2k+1-1] + x[j·2k+1-2k-1]
endfor
Simple modifications to current memory addressing scheme can save a lot of cycles 29

30. Bank Conflicts
Occur when multiple threads access the same shared memory bank with different addresses
In our case, we have something like 2k+1·j-1
k=0: two way bank conflict
k=1: four way bank conflict …
No penalty if all threads access different banks
Or if all threads access exact same address
Recall that shared memory accesses with conflicts are serialized
N-bank memory conflicts lead to a set of N successive shared memory transactions 30

31. Initial Bank Conflicts on Load
Each thread loads two shared mem data elements
Tempting to interleave the loads (see lines 9 & 10, and 46 & 47)
temp[2*thid]   = g_idata[2*thid];
temp[2*thid+1] = g_idata[2*thid+1];
Thread 0 accesses banks 0 and 1
Thread 1 accesses banks 2 and 3 …
Thread 8 accesses banks 16 and 17. Oops, that’s 0 and 1 again…
Two way bank conflict, can’t be easily eliminated
Better to load one element from each half of the array
temp[thid]         = g_idata[thid];
temp[thid + (n/2)] = g_idata[thid + (n/2)];
(Dan): Threads:(0,1,2,…,8,9,10,…)banks:(0,2,4,…,0,2,4,…) – I don’t think it’s right… 31
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32. Bank Conflicts in the tree algorithm
When we build the sums, during the first iteration of the algorithm each thread in a half-warp reads two shared memory locations and writes one:
Th(0,8) access bank 0
Bank: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
... … … T0 T1 T2 T3 T4 T5 T6 T7 T8 T9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
... …
First iteration: 2 threads access each of 8 banks.
Each corresponds to a single thread.
Like-colored arrows represent simultaneous memory accesses 32
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33. Bank Conflicts in the tree algorithm
When we build the sums, each thread reads two shared memory locations and writes one:
Th(1,9) access bank 2, etc.
Bank: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
... … … T0 T1 T2 T3 T4 T5 T6 T7 T8 T9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
... …
First iteration: 2 threads access each of 8 banks.
Each corresponds to a single thread.
Like-colored arrows represent simultaneous memory accesses 33
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34. Bank Conflicts in the tree algorithm
2nd iteration: even worse!
4-way bank conflicts; for example:
Th(0,4,8,12) access bank 1, Th(1,5,9,13) access Bank 5, etc.
Bank: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
... … … T0 T1 T2 T3 T4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
... 16 …
2nd iteration: 4 threads access each of 4 banks.
Each corresponds to a single thread.
Like-colored arrows represent simultaneous memory accesses 34
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35. Managing Bank Conflicts in the Tree Algorithm
Use padding to prevent bank conflicts
Add a word of padding every 16 words.
Now you work with a virtual 17 bank shared memory layout
Within a 16-thread half-warp, all threads access different banks
They are aligned to a 17 word memory layout
It comes at a price: you have memory words that are wasted
Keep in mind: you should also load data from global into shared memory using the virtual memory layout of 17 banks 35

36. Use Padding to Reduce Conflicts
After you compute a ShMem address like this:
Address = 2 * stride * thid;
Add padding like this: Address += (address >> 4); // divide by NUM_BANKS
This removes most bank conflicts
Not all, in the case of deep trees
Material posted online will contain a discussion of this “deep tree” situation along with a proposed solution 36
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37. Managing Bank Conflicts in the Tree Algorithm
Original scenario.
Bank: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
... … … T0 T1 T2 T3 T4 T5 T6 T7 T8 T9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
... …
Actual physical memory (true bank number)
Modified scenario, virtual 17 bank memory layout.
(0)
(1)
(2)
(3)
Virtual Bank: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 … P … T0 T1 T2 T3 T4 T5 T6 T7 T8 T9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
... P … 37
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Note that only arrows with the same color happen simultaneously.

38. Concluding Remarks, Parallel Scan
Intuitively, the scan operation is not the type of procedure ideally suited for parallel computing
Even if it doesn’t fit like a glove, leads to nice speedup:
# elements
CPU Scan (ms)
GPU Scan (ms)
Speedup
1024
0.03
32768
0.68
65536
1.07
131072
3.63
262144
4.45
524288
4.66
5.28
5.64
5.87
5.93
5.98
Source: 2007 paper of Harris, Sengupta, Owens 38

39. Concluding Remarks, Parallel Scan
The Hillis-Steele (HS) implementation is simple, but suboptimal
The Harris-Sengupta-Owen (HSO) solution is convoluted, but scales like O(n)
The complexity of the algorithm due to an acute bank-conflict situation
Michael Garland argues that there is a lot of serendipity in the HSO algorithm, and it’s not clear if it’s worth pursuing
Finally, we have not solved the problem yet: we only looked at the case when our array has up to 1024 elements
You will have to think how to handle the 16,777,216=224 elements case
Likewise, it would be fantastic if you implement as well the case when the number of elements is not a power of 2 39

40. Taking it one step further: completely eliminating the bank conflicts
(individual reading) 40

41. Scan Bank Conflicts (1) A full binary tree with 64 leaf nodes:
Multiple 2-and 4-way bank conflicts
Shared memory cost for whole tree
1 32-thread warp = 6 cycles per thread w/o conflicts
Counting 2 shared mem reads and one write (s[a] += s[b])
6 * ( ) = 102 cycles
36 cycles if there were no bank conflicts (6 * 6)
32-thread warp normally takes 6 cycles for the line s[a] += s[b].
This line should become 3 instructions:
G2R rA, g[a]
ADD rA, g[b], rA
R2G g[a], rA
For 32 threads, these instructions take 2 cycles each. Let’s assume that having fewer than 32 active threads does not reduce this cycle count per instruction.
Without bank conflicts, it’s 6 cycles total, times 6 levels of the tree = 36 cycles.
With bank conflicts it’s
6*( ) = 102 cycles 41

42. Scan Bank Conflicts (2) It’s much worse with bigger trees!
A full binary tree with 128 leaf nodes
Only the last 6 iterations shown (root and 5 levels below)
Cost for whole tree:
12*2 + 6*( ) = 186 cycles
48 cycles if there were no bank conflicts! 12*1 + (6*6)
For 32 threads, these instructions take 2 cycles each. Let’s assume that having fewer than 32 active threads does not reduce this cycle count per instruction.
Without bank conflicts, it’s 6 cycles total, times 7 levels of the tree = 42 cycles (Assuming that both 32-thread warps on the lowest level (64 threads to process 128 elements) run simultaneously).
With bank conflicts it’s
6*( ) = 174 cycles
WMH: I believe that the first 2 waves of accesses also have 8-way conflicts. They should take 8 rounds to resolve. 42

43. Scan Bank Conflicts (3) A full binary tree with 512 leaf nodes
Only the last 6 iterations shown (root and 5 levels below)
Cost for whole tree:
48*2+24*4+12*8+6* ( ) = 570 cycles
120 cycles if there were no bank conflicts!
For 32 threads, these instructions take 2 cycles each. Let’s assume that having fewer than 32 active threads does not reduce this cycle count per instruction.
Without bank conflicts, it’s 6 cycles total, times 9 levels of the tree = 54 cycles (Assuming that all 8 32-thread warps on the lowest level (256 threads to process 512 elements) run simultaneously).
With bank conflicts it’s
6*( ) = 366 cycles
WMH: Again, the first three should be all 16-way conflicts. 43

44. Fixing Scan Bank Conflicts
Insert padding every NUM_BANKS elements
const int LOG_NUM_BANKS = 4; // 16 banks on G80
int tid = threadIdx.x;
int s = 1;
// Traversal from leaves up to root
for (d = n>>1; d > 0; d >>= 1) {
if (thid <= d)
int a = s*(2*tid); int b = s*(2*tid+1)
a += (a >> LOG_NUM_BANKS); // insert pad word
b += (b >> LOG_NUM_BANKS); // insert pad word
shared[a] += shared[b]; } 44

45. Fixing Scan Bank Conflicts
A full binary tree with 64 leaf nodes
No more bank conflicts!
However, there are ~8 cycles overhead for addressing
For each s[a] += s[b] (8 cycles/iter. * 6 iter. = 48 extra cycles)
So just barely worth the overhead on a small tree
84 cycles vs. 102 with conflicts vs. 36 optimal
2 shifts and 2 adds per address computation. At 2 cycles each for a 32-thread warp, that’s 8 cycles overhead, plus the 6 cycles for the s[a] += s[b] without bank conflicts. So
(6+8)*6 = 84 cycles 45

46. Fixing Scan Bank Conflicts
A full binary tree with 128 leaf nodes
Only the last 6 iterations shown (root and 5 levels below)
No more bank conflicts!
Significant performance win:
106 cycles vs. 186 with bank conflicts vs. 48 optimal
1 shift and 1 add per address computation. At 2 cycles each for a 32-thread warp, that’s 8 cycles overhead, plus the 6 cycles for the s[a] += s[b] without bank conflicts. So
(6+8)*7 = 98 cycles 46

47. Fixing Scan Bank Conflicts
A full binary tree with 512 leaf nodes
Only the last 6 iterations shown (root and 5 levels below)
Wait, we still have bank conflicts
Method is not foolproof, but still much improved
304 cycles vs. 570 with bank conflicts vs. 120 optimal
1 shift and 1 add per address computation. At 2 cycles each for a 32-thread warp, that’s 8 cycles overhead plus the 6 cycles for the s[a] += s[b] without bank conflicts.
But we still have 2-way bank conflicts on 4 tree levels (out of 9 total). So…
(6+8)*5 + (12+8)*4= 150 cycles 47

48. Fixing Scan Bank Conflicts
It’s possible to remove all bank conflicts
Just do multi-level padding
Example: two-level padding:
const int LOG_NUM_BANKS = 4; // 16 banks on G80
int tid = threadIdx.x;
int s = 1;
// Traversal from leaves up to root
for (d = n>>1; d > 0; d >>= 1) {
if (thid <= d)
int a = s*(2*tid); int b = s*(2*tid+1)
int offset = (a >> LOG_NUM_BANKS); // first level
a += offset + (offset >>LOG_NUM_BANKS); // second level
offset = (b >> LOG_NUM_BANKS); // first level
b += offset + (offset >>LOG_NUM_BANKS); // second level
temp[a] += temp[b]; }
A and b calculation so that both offset and offset>>LOG_NUM_BANKS are added to them. 48

49. Fixing Scan Bank Conflicts
A full binary tree with 512 leaf nodes
Only the last 6 iterations shown (root and 5 levels below)
No bank conflicts
But an extra cycle overhead per address calculation
Not worth it: 440 cycles vs. 304 with 1-level padding
With 1-level padding, bank conflicts only occur in warp 0
Very small remaining cost due to bank conflicts
Removing them hurts all other warps
2 shifts and 1 add per address computation. At 2 cycles each for a 32-thread warp, that’s 12 cycles overhead, plus the 6 cycles for the s[a] += s[b] without bank conflicts.
(6+12)*9 = 162 cycles 49