data::cell targets 8KB as its maximum allocations size to avoid
pressuring the allocator. This 8KB target is used for internal storage
-- values small enough to be stored inside the cell itself -- as well
for external storage. Externally stored values use 8KB fragment sizes.
The problem is that only the size of data itself was considered when
making the allocations. For example when allocating the fragments
(chunks) for external storage, each fragment stored 8KB of data. But
fragments have overhead, they have next and back pointers. This resulted
in a 8KB + 2 * sizeof(void*) allocation. IMR uses the allocation
strategy mechanism, which works with aligned allocations. As the seastar
allocation only guarantees aligned allocations for power of two sizes,
it ends up allocating a 16KB slot. This results in the mutation fragment
using almost twice as much memory as would be required. This is a huge
waste.
This patch fixes the problem by considering the overhead of both
internal and external storage ensuring allocations are 8KB or less.
Fixes: #6043
Tests: unit(debug, dev, release)
Signed-off-by: Botond Dénes <bdenes@scylladb.com>
Message-Id: <20200910171359.1438029-1-bdenes@scylladb.com>
Seastar recently lost support for the experimental Concepts Technical
Specification (TS) and gained support for C++20 concepts. Re-enable
concepts in Scylla by updating our use of concepts to the C++20
standard.
This change:
- peels off uses of the GCC6_CONCEPT macro
- removes inclusions of <seastar/gcc6-concepts.hh>
- replaces function-style concepts (no longer supported) with
equation-style concepts
- semicolons added and removed as needed
- deprecated std::is_pod replaced by recommended replacement
- updates return type constraints to use concepts instead of
type names (either std::same_as or std::convertible_to, with
std::same_as chosen when possible)
No attempt is made to improve the concepts; this is a specification
update only.
Message-Id: <20200531110254.2555854-1-avi@scylladb.com>
Currently the loop which writes the data from the fragmented origin to
the destination, moves to the next chunk eagerly after writing the value
of the current chunk, if the current chunk is exhausted.
This presents a problem when we are writing the last piece of data from
the last chunk, as the chunk will be exhausted and we eagerly attempt to
move to the next chunk, which doesn't exist and dereferencing it will
fail. The solution is to not be eager about moving to the next chunk and
only attempt it if we actually have more data to write and hence expect
more chunks.
This commit introduces cell serializers and views based on the in-memory
representation infrastructure. The code doesn't assume anything about
how the cells are stored, they can be either a part of another IMR
object (once the rows are converted to the IMR) or a separate objects
(just like current atomic_cell).
