Avi Kivity 6d9e18fd61 logalloc: reduce descriptor overhead ...

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.

2017-04-24 12:23:12 +02:00

7.5 KiB

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