dataset.readerDownloader was originally introduced in #16429, an attempt to
balance peak memory usage of reading a section with read times by downloading a
configurable size of pages in advance.
In practice, each roundtrip to object storage adds too much of a latency hit,
and we've started to set the cache limit high enough to ensure that each reader
only needs a single prefetch. Given what we've found, it no longer makes sense
to control peak memory usage via the prefetch size. Other options, such as
downloading directly to disk, may be explored in the future.
In the meantime, this PR removes the ability to specify a cache size. All
non-pruned pages will be bulk requested using the range reader (#19067) on the
first read call. Pages which have left the potential read window will continue
to be eagerly removed for garbage collection.
However, we don't want to prefetch when the dataset is entirely in memory,
which is the case when the logs section builder is performing k-way merge over
in-memory sections. To lower the memory usage of builders, prefetching is
configurable. For this initial PR, prefetching is only disabled for the logs
section builder; all other reads force prefetching.
Signed-off-by: Robert Fratto <robertfratto@gmail.com>
