[haiku-bugs] [Haiku] #9255: BFS uses unaligned memory access for BPlusTree data
- From: "mmlr" <trac@xxxxxxxxxxxx>
- Date: Sun, 02 Dec 2012 13:27:27 -0000
#9255: BFS uses unaligned memory access for BPlusTree data
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Reporter: mmlr | Owner: axeld
Type: bug | Status: new
Priority: normal | Milestone: R1
Component: File Systems/BFS | Version: R1/Development
Keywords: | Blocked By:
Blocking: | Has a Patch: 0
Platform: All |
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The BPlusTrees in BFS pack their data (key lengths and values) tightly on
disk. These structures are then loaded into memory and used directly.
Their offsets are calculated, a pointer is formed and the memory is
accessed as an array. Since there is no alignment enforced, this can lead
to unaligned access of the uint16 key lengths or off_t values. This is
illegal from a language point of view, as pointers to a basic type are
required to be aligned to the size of that type (so 8 bytes for the
off_t). In practice this works on x86 as there unaligned access is just
handled silently. However this breaks on processors that have these
alignment restrictions like ARM (at least < v6).
As unaligned memory access has to be broken up into multiple bus accesses
behind the scenes, they do carry a performance penalty in any case and
should therefore be avoided.
To get further with the ARM port I've worked around the issue by
implementing getters and setters for the array access and implementing the
actual access through memcpy() that does not exhibit the alignment
problem. As this introduces a rather large overhead for commonly used
functionality in BFS, I am not going to apply this patch. I open this
ticket to document the issue and possibly come up with solutions that
don't carry as much overhead.
--
Ticket URL: <http://dev.haiku-os.org/ticket/9255>
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