Is Parallel Programming Hard, And, If So, What Can You Do About It?

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38 CHAPTER 4. COUNTING1 unsigned long __thread counter = 0;2 unsigned long __thread countermax = 0;3 unsigned long globalcountmax = 10000;4 unsigned long globalcount = 0;5 unsigned long globalreserve = 0;6 unsigned long *counterp[NR_THREADS] = { NULL };7 DEFINE_SPINLOCK(gblcnt_mutex);8 #define MAX_COUNTERMAX 100Figure 4.13: Approximate Limit Counter Variables1 static void balance_count(void)2 {3 countermax = globalcountmax - globalcount - globalreserve;4 countermax /= num_online_threads();5 if (countermax > MAX_COUNTERMAX)6 countermax = MAX_COUNTERMAX;7 globalreserve += countermax;8 counter = countermax / 2;9 if (counter > globalcount)10 counter = globalcount;11 globalcount -= counter;12 }Figure 4.14: Approximate Limit Counter Balancingof lines 5 and 6, which enforce the MAX_COUNTERMAXlimit on the per-thread countermax variable.1 atomic_t __thread counterandmax = ATOMIC_INIT(0);2 unsigned long globalcountmax = 10000;3 unsigned long globalcount = 0;4 unsigned long globalreserve = 0;5 atomic_t *counterp[NR_THREADS] = { NULL };6 DEFINE_SPINLOCK(gblcnt_mutex);7 #define CM_BITS (sizeof(atomic_t) * 4)8 #define MAX_COUNTERMAX ((1 > CM_BITS) & MAX_COUNTERMAX;14 *cm = cami & MAX_COUNTERMAX;15 }1617 static void18 split_counterandmax(atomic_t *cam, int *old,19 int *c, int *cm)20 {21 unsigned int cami = atomic_read(cam);2223 *old = cami;24 split_counterandmax_int(cami, c, cm);25 }2627 static int merge_counterandmax(int c, int cm)28 {29 unsigned int cami;3031 cami = (c
4.4. EXACT LIMIT COUNTERS 39imum value that may be held in either half ofcounterandmax.Quick Quiz 4.29: In what way does line 7 ofFigure 4.15 violate the C standard?Lines 10-15 show the split_counterandmax_int() function, which, when given the underlyingint from the atomic_t counterandmax variable.Line 13 isolates the most-significant half of this int,placing the result as specified by argument c, andline 14 isolates the least-significant half of this int,placing the result as specified by argument cm.Lines 17-25 show the split_counterandmax()function, which picks up the underlying int fromthe specified variable on line 21, stores it as specifiedbytheoldargumentonline23,andtheninvokessplit_counterandmax_int() to split it on line 24.Quick Quiz 4.30: Given that there is only onecounterandmax variable, why bother passing in apointer to it on line 18 of Figure 4.15?Lines 27-33 show the merge_counterandmax()function, which can be thought of as the inverseof split_counterandmax(). Line 31 merges thecounter and countermax values passed in c and cm,respectively, and returns the result.Quick Quiz 4.31: Why does merge_counterandmax() in Figure 4.15 return an intrather than storing directly into an atomic_t?Figure 4.16 shows the add_count(), sub_count(), and read_count() functions.Lines 1-32 show add_count(), whose fastpathspans lines 8-15, with the remainder of the functionbeing the slowpath. Lines 8-14 of the fastpathform a compare-and-swap (CAS) loop, withtheatomic_cmpxchg()primitivesonlines13-14performingthe actual CAS. Line 9 splits the currentthread’s counterandmax variable into its counter(in c) and countermax (in cm) components, whileplacing the underlying int into old. Line 10 checkswhether the amount delta can be accommodatedlocally (taking care to avoid integer overflow), andif not, line 11 transfers to the slowpath. Otherwise,line 11 combines an updated counter valuewith the original countermax value into new. Theatomic_cmpxchg() primitive on lines 13-14 thenatomically compares this thread’s counterandmaxvariable to old, updating its value to new if thecomparison succeeds. <strong>If</strong> the comparison succeeds,line 15 returns success, otherwise, execution continuesin the loop at line 9.Quick Quiz 4.32: Yecch!!! Why the ugly gotoon line 11 of Figure 4.16? Haven’t you heard of thebreak statement???Quick Quiz 4.33: Why would the atomic_cmpxchg() primitive at lines 13-14 of Figure 4.161 int add_count(unsigned long delta)2 {3 int c;4 int cm;5 int old;6 int new;78 do {9 split_counterandmax(&counterandmax, &old, &c, &cm);10 if (delta > MAX_COUNTERMAX || c + delta > cm)11 goto slowpath;12 new = merge_counterandmax(c + delta, cm);13 } while (atomic_cmpxchg(&counterandmax,14 old, new) != old);15 return 1;16 slowpath:17 spin_lock(&gblcnt_mutex);18 globalize_count();19 if (globalcountmax - globalcount -20 globalreserve < delta) {21 flush_local_count();22 if (globalcountmax - globalcount -23 globalreserve < delta) {24 spin_unlock(&gblcnt_mutex);25 return 0;26 }27 }28 globalcount += delta;29 balance_count();30 spin_unlock(&gblcnt_mutex);31 return 1;32 }3334 int sub_count(unsigned long delta)35 {36 int c;37 int cm;38 int old;39 int new;4041 do {42 split_counterandmax(&counterandmax, &old, &c, &cm);43 if (delta > c)44 goto slowpath;45 new = merge_counterandmax(c - delta, cm);46 } while (atomic_cmpxchg(&counterandmax,47 old, new) != old);48 return 1;49 slowpath:50 spin_lock(&gblcnt_mutex);51 globalize_count();52 if (globalcount < delta) {53 flush_local_count();54 if (globalcount < delta) {55 spin_unlock(&gblcnt_mutex);56 return 0;57 }58 }59 globalcount -= delta;60 balance_count();61 spin_unlock(&gblcnt_mutex);62 return 1;63 }Figure 4.16: Atomic Limit Counter Add and Subtract
Page 1 and 2: Is Parallel Programming Hard, And,
Page 3 and 4: Contents1 Introduction 11.1 Histori
Page 5 and 6: CONTENTSv6 Locking 676.1 Staying Al
Page 7 and 8: CONTENTSviiB Synchronization Primit
Page 9 and 10: CONTENTSixE.7.1 Introduction to Pre
Page 11 and 12: PrefaceThe purpose of this book is
Page 13 and 14: Chapter 1IntroductionParallel progr
Page 15 and 16: 1.2. PARALLEL PROGRAMMING GOALS 3CP
Page 17 and 18: 1.3. ALTERNATIVES TO PARALLEL PROGR
Page 19 and 20: 1.4. WHAT MAKES PARALLEL PROGRAMMIN
Page 21 and 22: 1.5. GUIDE TO THIS BOOK 9other hand
Page 23 and 24: Chapter 2Hardware and its HabitsMos
Page 25: 2.1. OVERVIEW 13Therefore, as shown
Page 28 and 29: 16 CHAPTER 2. HARDWARE AND ITS HABI
Page 30 and 31: 18 CHAPTER 2. HARDWARE AND ITS HABI
Page 32 and 33: 20 CHAPTER 3. TOOLS OF THE TRADE1 p
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Page 36 and 37: 24 CHAPTER 3. TOOLS OF THE TRADE1.1
Page 38 and 39: 26 CHAPTER 3. TOOLS OF THE TRADEQui
Page 40 and 41: 28 CHAPTER 3. TOOLS OF THE TRADE
Page 42 and 43: 30 CHAPTER 4. COUNTING1 atomic_t co
Page 44 and 45: 32 CHAPTER 4. COUNTING4.2.3 Eventua
Page 46 and 47: 34 CHAPTER 4. COUNTINGvanish when t
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Page 54 and 55: 42 CHAPTER 4. COUNTING1 #define THE
Page 56 and 57: 44 CHAPTER 4. COUNTING1 unsigned lo
Page 58 and 59: 46 CHAPTER 4. COUNTINGReadsAlgorith
Page 60 and 61: 48 CHAPTER 5. PARTITIONING AND SYNC
Page 80 and 81: 68 CHAPTER 6. LOCKING1 int delete(i
Page 82 and 83: 70 CHAPTER 7. DATA OWNERSHIP
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110 CHAPTER 9. APPLYING RCU1 struct
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112 CHAPTER 9. APPLYING RCU
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114 CHAPTER 10. VALIDATION: DEBUGGI
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116 CHAPTER 11. DATA STRUCTURES
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118 CHAPTER 12. ADVANCED SYNCHRONIZ
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138 CHAPTER 13. EASE OF USEFigure 1
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140 CHAPTER 13. EASE OF USE
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142 CHAPTER 14. TIME MANAGEMENT
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144 CHAPTER 15. CONFLICTING VISIONS
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154 APPENDIX A. IMPORTANT QUESTIONS
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156 APPENDIX A. IMPORTANT QUESTIONS
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158 APPENDIX B. SYNCHRONIZATION PRI
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160 APPENDIX B. SYNCHRONIZATION PRI
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162 APPENDIX C. WHY MEMORY BARRIERS
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184 APPENDIX D. READ-COPY UPDATE IM
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244 APPENDIX E. FORMAL VERIFICATION
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272 APPENDIX F. ANSWERS TO QUICK QU
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330 APPENDIX G. GLOSSARY(2) A physi
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332 APPENDIX G. GLOSSARYnear by. Th
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334 APPENDIX G. GLOSSARY
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336 BIBLIOGRAPHY[But97]USA, March 2
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338 BIBLIOGRAPHY[HMB06][Hol03][HP95
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340 BIBLIOGRAPHY[McK06] Paul E. McK
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342 BIBLIOGRAPHYtor. Software - Pra
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344 BIBLIOGRAPHY[UoC08][VGS08]Berke
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346 APPENDIX H. CREDITSH.4 Original
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Is Parallel Programming Hard, And, If So, What Can You Do About It?

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