Duarte Nunes 11b5076b3c lsa: Use log histogram for closed segments ...

This patch replaces the current heap with a logarithmic histogram
to hold the closed segment descriptors.

This histogram stores elements in different buckets according to
their size. Values are mapped to a sequence of power-of-two ranges
that are split in N sub-buckets. Values less than a minimum value
are placed in bucket 0, whereas values bigger than a maximum value
are not admitted.

There is some loss of precision as segments are now not totally
ordered, and precision decreases the more sparse a segment is. This
allows to reduce the cost of the computations needed when freeing
from a closed segment.

Performance results for perf_simple_query -c4 --duration 60
           before       after       diff
read     43954.27    45246.10      +2.9%
write    48911.54    52807.76      +7.9%

Fixes #1442

Signed-off-by: Duarte Nunes <duarte@scylladb.com>
Message-Id: <20170227235328.27937-1-duarte@scylladb.com>

2017-02-28 18:40:38 +02:00

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