Cache all cids in memory in pruner

[hannahhoward]

Goals

Performance testing indicates the pruner is still a huge bottleneck on performance, and the hot spot is reading the all keys chan.

By our calculations, with not too much penalty we can keep a list of all keys in memory, and thus avoid this bottleneck.

Implementation

• instead of tracking pins, track all cids including whether they are pinned
• periodically sync with disk
• use a sync.Pool to avoid unneeded allocations

For discussion

This is complicated enough I want to write tests before anyone merges it, but want folks to review the approach now.

[codecov-commenter]

Codecov Report

Base: 5.43% // Head: 5.31% // Decreases project coverage by -0.12% ⚠️

Coverage data is based on head (a1610e9) compared to base (27eabc8).
Patch coverage: 0.00% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff            @@
##           master    #175      +/-   ##
=========================================
- Coverage    5.43%   5.31%   -0.12%     
=========================================
  Files          14      14              
  Lines        1639    1674      +35     
=========================================
  Hits           89      89              
- Misses       1545    1580      +35     
  Partials        5       5              

Impacted Files	Coverage Δ
blocks/randompruner.go	0.00% <0.00%> (ø)

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[rvagg]

I still hate this file-writing business. I reckon now it'd be more efficient, and safe, to just iterate over the CID map and roll the dice on each one, with some probability of 1% or something small. Delete when they lose the dice roll and aren't pinned, stop iterating if we reach our threshold, or iterate again if we haven't (maybe with some safety around it like don't loop more than 20 times). Go's unstable map iteration ordering even helps a bit here.

[elijaharita]

i didn't take the effort to estimate how much memory cids would take up when i wrote this. in my head it sounded big. but u are right, with a blockstore target size of around 100GiB it seems fairly negligible, a server who wants bigger cache should have plenty of memory available too.

if we do keep all the cids in memory, it opens the door for a much less dumb pruner solution - fifo? doesn't need to be implemented now but probably should do at some point.

[elijaharita]

and i think @rvagg is right, file io definitely should go away now if keys are in memory already