Indexed select statements fetch primary key information from
their internal materialized views and then use it to query
the base table. Unfortunately, the current mechanism for retrieving
base table rows makes it easy to overwhelm the replicas with unbounded
concurrency - the number of concurrent ops is increased exponentially
until a short read is encountered, but it's not enough to cap the
concurrency - if data is fetched row-by-row, then short reads usually
don't occur and as a result it's easy to see concurrency of 1M or
higher. In order to avoid overloading the replicas, the concurrency
of indexed queries is now capped at 4096 and additionally throttled
if enough results are already fetched. For paged queries it means that
the query returns as soon as 1MB of data is ready, and for unpaged ones
the concurrency will no longer be doubled as soon as the previous
iteration fetched 1MB of results.
The fixed 4096 value can be subject to debate, its reasoning is as follows:
for 2KiB rows, so moderately large but not huge, they result in
fetching 10MB of data, which is the granularity used by replicas.
For 200B rows, which is rather small, the result would still be
around 1MB.
At the same time, 4096 separate tasks also means 4096 allocations,
so increasing the number also strains the allocator.
Fixes#8799
Tests: unit(release),
manual: observing metrics of modified index_paging_test
Closes#8814
* github.com:scylladb/scylla:
cql3: limit the transitional result size for indexed queries
cql3: return indexed pages after 1MB worth of data
cql3: limit the concurrency of indexed statements
3e3003fcc1