[go-nuts] Why does scavChunkData use a 1-bit flag instead of a 10-bit counter to record whether free pages exist?

[Lidong Yan]

Hi everyone, I am a newcomer reading Go runtime code, and I noticed that
```go
type scavChunkData struct {
// Only the first 10 bits are used.
inUse uint16

// Only the first 10 bits are used.
lastInUse uint16

// gen is the generation counter from a scavengeIndex from the
// last time this scavChunkData was updated.
gen uint32

// Note: only 6 bits are available.
scavChunkFlags
}
```

in scavChunkData, we only use 1-bit of scavChunkFlags to indicate whether 
there is free pages in a chunk. This could be imprecise because we use the 
following code to compute whether there are free pages,
```go
nofree = inUse == pagesInChunk
```

I think we could use a 10-bit counter to record how many pages are freed. 
Why go use a 1-bit flag instead of a 10-bit counter? Is it for performance 
reasons?

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