keep allocations as small as possible

[wyndhblb]

Under some large streaming volume conditions (kafka consumption) this allocation can create tremendous GC pressure, where setting GOGC <= 10 still is not enough. Since this condition does not happen "very often" (but seems to happen more often in this streaming case then originally advertised by the original comment) we only allocate as much as we need not an extra 2Mb slab.

Conditions:

• 2000 zstd encoded blocks second over 64 different streams byte blocks were in 1Mb chunks

Before this change:

• with in several seconds Ram consumption would jump from 200Mb to over 30Gb
• pprof indicated this allocation as the issue, however only ~500Mb inuse indicating GC had not cleaned up the other allocs
• turning on GOGC=5, stemmed the speed of OOM, but still eventually OOMed

After this change:

• ram usage is steady at around 200Mb and message rate seems same as before.

[wyndhblb]

see --> IBM/sarama#1831

[klauspost]

Suggested change

	addBytes := 1024 + (size - len(s.out))
	used := len(s.out) - startSize
	addBytes := 256 + ll + ml + used>>2
	// Clamp to max block size.
	if used+addBytes > maxBlockSize {
	addBytes = maxBlockSize - used
	}

[klauspost]

Maybe something like this?

[wyndhblb]

ok gave that change a whirl
there's about a 500-750Mb difference in the ram usage, higher using method, then the original size - len one, but it does seem to not get out of control, which is the desired effect.

[klauspost]

Great. I will merge when Ci tests are done and probably release it tomorrow.