sstable: remove iterstatsaccumulator and refactor

[EdwardX29]

Removed block.IterStatsAccumulator since there is only one implementation of the interface. Refactored usages of block.IterStatsAccumulator to use CategoryStatsWithMu. Also, increased numCategoryStatsShards to be runtime.GOMAXPROCS(0).

[cockroach-teamcity]

This change is 

[sumeerbhola]

Reviewed 3 of 3 files at r1.
Reviewable status: all files reviewed (commit messages unreviewed), 2 unresolved discussions (waiting on @aadityasondhi, @EdwardX29, and @jbowens)

---

sstable/block/category_stats.go line 173 at r1 (raw file):

}

var numCategoryStatsShards = runtime.GOMAXPROCS(0)

how about something like max(16, runtime.GOMAXPROCS(0)).
Don't want 2 shards if someone does something silly and sets GOMAXPROCS to 2.

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sstable/block/category_stats.go line 206 at r1 (raw file):

		CategoryStatsWithMu
		// Pad each shard to 64 bytes so they don't share a cache line.
		_ [64 - unsafe.Sizeof(CategoryStatsWithMu{})]byte

nit: can you lift this 64 - unsafe.Sizeof(CategoryStatsWithMu{} to a file-level const and use it here and below.

[RaduBerinde]

Reviewable status: 2 of 3 files reviewed, 4 unresolved discussions (waiting on @aadityasondhi, @EdwardX29, @jbowens, and @sumeerbhola)

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sstable/block/category_stats.go line 232 at r1 (raw file):

		v, _ = c.statsMap.LoadOrStore(category, &shardedCategoryStats{
			Category: category,
			shards: make([]struct {

[nit] define a type for this

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sstable/block/category_stats.go line 242 at r1 (raw file):

	// This equation is taken from:
	// https://en.wikipedia.org/wiki/Linear_congruential_generator#Parameters_in_common_use
	shard := ((p * 25214903917) >> 32) & uint64(numCategoryStatsShards-1)

This code assumes numCategoryStatsShards is a power of two; this is not necessarily the case with GOMAXPROCS. I'd also add this requirement as a comment on numCategoryStatsShards.

[RaduBerinde]

modulo some nits

Reviewable status: 2 of 3 files reviewed, 8 unresolved discussions (waiting on @aadityasondhi, @EdwardX29, @jbowens, and @sumeerbhola)

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sstable/block/category_stats.go line 40 at r4 (raw file):

// CategoryShard is a single shard of a category's statistics.
type CategoryShard struct {

categoryShard (doesn't need to be exported)

---

sstable/block/category_stats.go line 182 at r4 (raw file):

}

// numCategoryStatsShards should be a power of 2 to compute shard indices

[nit] I'd clean up the phrasing as such:

numCategoryStatsShards must be a power of 2. We initialize it to GOMAXPROCS
(rounded up to the nearest power of 2) or 16, whichever is larger.

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sstable/block/category_stats.go line 193 at r4 (raw file):

}()

type CategoryStatsWithMu struct {

This needs a comment. Also the name is not very useful, I think calling it CategoryStatsShard might make more sense (and name the container struct paddedCategoryStatsShard).

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sstable/block/category_stats.go line 194 at r4 (raw file):

type CategoryStatsWithMu struct {
	mu sync.Mutex

Usually we do

type X struct {
  mu struct {
    sync.Mutex
    stats CategoryStats
  }
}