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cache,db: de-dup concurrent attempts to read the same block #4157

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cache,db: de-dup concurrent attempts to read the same block #4157

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Commits on Nov 26, 2024

  1. cache,db: de-dup concurrent attempts to read the same block 

    Concurrent reads of the same block have been observed to cause very high
memory usage, and cause significant CPU usage for allocations/deallocations.
We now coordinate across multiple concurrent attempts to read the same
block via a readEntry, which makes the readers take turns until one
succeeds.

The readEntries are embedded in a map that is part of a readShard, where
there is a readShard for each cache.Shard. See the long comment in the
readShard declaration for motivation. The Options.LoadBlockSema is
integrated into the readEntry, to simplify the waiting logic in the
caller.

Callers interact with this new behavior via Cache.GetWithReadHandle,
which is only for callers that intend to do a read and then populate
the cache. If this method returns a ReadHandle, the caller has permission
to do a read. See the ReadHandle comment for details of the contract.

Fixes cockroachdb#4138
    @sumeerbhola
    sumeerbhola committed Nov 26, 2024
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