pre_text: (c) Summary of Share-Based Compensation Expense Share-based compensation expense consists primarily of expenses for stock options, stock purchase rights, restricted stock, and RSUs granted to employees. The following table summarizes share-based compensation expense (in millions): As of July 27, 2019, the total compensation cost related to unvested share-based awards not yet recognized was $3.3 billion, which is expected to be recognized over approximately 2.8 years on a weighted-average basis.
table: Years Ended July 27, 2019 July 28, 2018 July 29, 2017
table: Cost of sales—product $90 $94 $85
table: Cost of sales—service 130 133 134
table: Share-based compensation expense in cost of sales . 220 227 219
table: Research and development . 540 538 529
table: Sales and marketing 519 555 542
table: General and administrative 250 246 236
table: Restructuring and other charges 62 33 3
table: Share-based compensation expense in operating expenses 1,371 1,372 1,310
table: Total share-based compensation expense $1,591 $1,599 $1,529
table: Income tax benefit for share-based compensation . $542 $558 $451
question: Analyse this data from a financial earnings document. What was the change in the product cost of sales between 2017 and 2018?