Every lsa-allocated object is prefixed by a header that contains information
needed to free or migrate it. This includes its size (for freeing) and
an 8-byte migrator (for migrating). Together with some flags, the overhead
is 14 bytes (16 bytes if the default alignment is used).
This patch reduces the header size to 1 byte (8 bytes if the default alignment
is used). It uses the following techniques:
- ULEB128-like encoding (actually more like ULEB64) so a live object's header
can typically be stored using 1 byte
- indirection, so that migrators can be encoded in a small index pointing
to a migrator table, rather than using an 8-byte pointer; this exploits
the fact that only a small number of types are stored in LSA
- moving the responsibility for determining an object's size to its
migrator, rather than storing it in the header; this exploits the fact
that the migrator stores type information, and object size is in fact
information about the type
The patch improves the results of memory_footprint_test as following:
Before:
- in cache: 976
- in memtable: 947
After:
mutation footprint:
- in cache: 880
- in memtable: 858
A reduction of about 10%. Further reductions are possible by reducing the
alignment of lsa objects.
logalloc_test was adjusted to free more objects, since with the lower
footprint, rounding errors (to full segments) are different and caused
false errors to be detected.
Missing: adjustments to scylla-gdb.py; will be done after we agree on the
new descriptor's format.
Renaming the function to external_memory_usage() makes it clear that
sizeof(T) is not included, something that was a source of confusion in
the past.
Signed-off-by: Paweł Dziepak <pdziepak@scylladb.com>
gcc 6 complains that deleting a managed_bytes::external isn't defined
because the size isn't known. I'm not sure it's correct, but there's no
way to tell because flexible arrays aren't standardized.
Fix by using an array of zero size.
Message-Id: <1466715187-4125-1-git-send-email-avi@scylladb.com>
Clang enforces that a union's constexpr CTOR must initialize
one of the members. The spec is seemingly silent as to what
the rule on this is, however, making this non-constexpr results in clang
accepting the constructor.
Signed-off-by: Erich Keane <erich.keane@verizon.net>
Message-Id: <1454604300-1673-1-git-send-email-erich.keane@verizon.net>
The migrator tells lsa how to move an object when it is compacted.
Currently it is a function pointer, which means we must know how to move
the object at compile time. Making it an object allows us to build the
migration function at runtime, making it suitable for runtime-defined types
(such as tuples and user-defined types).
In the future, we may also store the size there for fixed-size types,
reducing lsa overhead.
C++ variable templates would have made this patch smaller, but unfortunately
they are only supported on gcc 5+.
Copying values may throw (std::bad_alloc for example), which will
result in a leak because destructor will not be called when
constructor throws.
May manifest as the following assertion failure:
utils/logalloc.cc:672: virtual logalloc::region_impl::~region_impl(): Assertion `_active->is_empty()' failed.
