Hello, I'm using btrfs on am ARMv7 and it turns out, that the kernel has to fixup a lot of kernel originated alignment issues.
See /proc/cpu/alignment (~4h of uptime): > System: 22304815 (btrfs_get_token_64+0x13c/0x148 [btrfs]) For example, when compiling the kernel on a btrfs volume the counter increases by 100...1000 per second. The function shown "btrfs_get_token_64()" is defined here: > http://lxr.free-electrons.com/source/fs/btrfs/struct-funcs.c#L53 ...it already uses get_unaligned_leXX accessors. Quoting a comment in arch/arm/mm/alignment.c: * ARMv6 and later CPUs can perform unaligned accesses for * most single load and store instructions up to word size. * LDM, STM, LDRD and STRD still need to be handled. But on a 32bit ARMv7 64bits are not word-sized. Is the exception and fixup overhead neglectable? Do we have to introduce something like HAVE_EFFICIENT_UNALIGNED_64BIT_ACCESS? regards, Marc -- Pengutronix e.K. | Marc Kleine-Budde | Industrial Linux Solutions | Phone: +49-231-2826-924 | Vertretung West/Dortmund | Fax: +49-5121-206917-5555 | Amtsgericht Hildesheim, HRA 2686 | http://www.pengutronix.de |
signature.asc
Description: OpenPGP digital signature
