repair: further limit parallelism of checksum calculation
repair: further limit parallelism of checksum calculation

Repair today has a semaphore limiting the number of ongoing checksum
comparisons running in parallel (on one shard) to 100. We needed this
number to be fairly high, because a "checksum comparison" can involve
high latency operations - namely, sending an RPC request to another node
in a remote DC and waiting for it to calculate a checksum there, and while
waiting for a response we need to proceed calculating checksums in parallel.

But as a consequence, in the current code, we can end up with as many as
100 fibers all at the same stage of reading partitions to checksum from
sstables. This requires tons of memory, to hold at least 128K of buffer
(even more with read-ahead) for each of these fibers, plus partition data
for each. But doing 100 reads in parallel is pointless - one (or very few)
should be enough.

So this patch adds another semaphore to limit the number of checksum
*calculations* (including the read and checksum calculation) on each shard
to just 2. There may still be 100 ongoing checksum *comparisons*, in
other stages of the comparisons (sending the checksum requests to other
and waiting for them to return), but only 2 will ever be in the stage of
reading from disk and checksumming them.

The limit of 2 checksum calculations (per shard) applies on the repair
slave, not just to the master: The slave may receive many checksum
requests in parallel, but will only actually work on 2 at a time.

Because the parallelism=100 now rate-limits operations which use very little
memory, in the future we can safely increase it even more, to support
situations where the disk is very fast but the link between nodes has
very high latency.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20170703151329.25716-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

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