2385fba4b6
This patch fixes one cause of oversized allocations - and therefore
potentially stalls and increased tail latencies - in Alternator.
Alternator's Scan or Query operation return a page of results. When the
number of items is not limited by a "Limit" parameter, the default is
to return a 1 MB page. If items are short, a large number of them can
fit in that 1MB. The test test_query.py::test_query_large_page_small_rows
has 30,000 items returned in a single page.
In the response JSON, all these items are returned in a single array
"Items". Before this patch, we build the full response as a RapidJSON
object before sending it. The problem is that unfortunately, RapidJSON
stores arrays as contiguous allocations. This results in large
contiguous allocations in workloads that scan many small items, and
large contiguous allocations can also cause stalls and high tail
latencies. For example, before this patch, running
test/alternator/run --runveryslow \
test_query.py::test_query_large_page_small_rows
reports in the log:
oversized allocation: 573440 bytes.
After this patch, this warning no longer appears.
The patch solves the problem by collecting the scanned items not in a
RapidJSON array, but rather in a chunked_vector<rjson::value>, i.e,
a chunked (non-contiguous) array of items (each a JSON value).
After collecting this array separately from the response object, we
need to print its content without actually inserting it into the object -
we add a new function print_with_extra_array() to do that.
The new separate-chunked-vector technique is used when a large number
(currently, >256) of items were scanned. When there is a smaller number
of items in a page (this is typical when each item is longer), we just
insert those items in the object and print it as before.
Beyond the original slow test that demonstrated the oversized allocation
(which is now gone), this patch also includes a new test which
exercises the new code with a scan of 700 (>256) items in a page -
but this new test is fast enough to be permanently in our test suite
and not a manual "veryslow" test as the other test.
Fixes #23535
19 KiB
19 KiB