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>
192bcc5811