Re: [patch] block layer: kmemcheck fixes

Arjan van de Ven
2008-02-08T22:57:16Z

Nick Piggin wrote:
>>> Maybe cpus these days have so much store bandwith that doing
>>> things like the above is OK, but I doubt it :-)
>> on modern x86 cpus the memset may even be faster if the memory isn't in
>> cache;
>> the "explicit" method ends up doing Write Allocate on the cache lines
>> (so read them from memory) even though they then end up being written
>> entirely.
>> With memset the CPU is told that the entire range is set to a new value, and
>> the WA can be avoided for the whole-cachelines in the range.
>
> Don't you have write combining store buffers? Or is it still speculatively
> issuing the reads even before the whole cacheline is combined?

x86 memory order model doesn't allow that quite; and you need a "series" of at least 64 bytes
without any other memory accesses in between even if it would....
not happening in practice.