maria-discuss team mailing list archive

[Marko Mäkelä <marko.makela@xxxxxxxxxxx>]

On Thu, Nov 28, 2019 at 3:24 PM Gordan Bobic <gordan.bobic@xxxxxxxxx> wrote:
> For what it's worth, unaligned access does come with a performance penalty, typically somewhere in the 1-10% range on x86, depending on the generation of chip used. It has been _mostly_ mitigated on recent x86 chips, and IIRC Intel's C compiler does have an option to align all structs and arrays to a 16 byte boundary.

Yes, there is overhead, and there are some unfortunate design choices
(or problems) with the InnoDB page format. Luckily, most page header
and footer fields are reasonably aligned.

> I would be very interested to see some tests data on unalighed access cost on various aarch64 chips. On various 32-bit ARM chips (including those >= ARMv6) the unaligned access performance hit was quite dramatic.

I wonder if the unaligned access could ever end up costing more than
the instruction decoding overhead for implementing multi-byte access
via single-byte operations. (In the past, when unaligned access could
have been supported by an interrupt to the operating system, like
Digital UNIX on the Alpha, I could easily believe it. But, now we are
talking about hardware-supported unaligned access.)

Marko
-- 
Marko Mäkelä, Lead Developer InnoDB
MariaDB Corporation