FreeBSD & Software RAID
wojtek at wojtek.tensor.gdynia.pl
Tue May 26 19:26:47 UTC 2009
> ZFS is thoroughly 64-bit and uses 64-bit math pervasively. That means
> have to emulate all those operations with 2 32-bit values, and on the
> register-starved x86 platform you end up with absolutely horrible
no this difference isn't that great. it doesn't use much less CPU on the
same processor using i386 and amd64 kernels - i checked it.
no precise measurements but there are no more than 20% performance
difference - comparable to most programs used in i386 and amd64 mode.
so no "horrible performance" on i386, or if you prefer - always horrible
performance no matter what CPU mode.
while x86 architecture doesn't have much registers
EAX,EBX,ECX,EDX,ESI,EDI,EBP,ESP 8 total (+EIP) it doesn't affect programs
that much, as all modern x86 processors perform memory-operand instructions
single cycle (or more than one of them).
anyway extra 8 registers and PC-relative addresses are very useful. this
roughly 20% performance difference is because of this.
if you mean gain on 64-bit registers when calculating block checksums in
ZFS - it's for sure memory-bandwidth and latency limited, not CPU power.
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