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SubjectRe: [patch] Prefetching file_read_actor()
Dave Jones wrote:
>
> After noticing file_read_actor() showing up in profiles quite
> a bit, I grepped old l-k messages, and turned up a post by
> Manfred Spraul in which he posted a patch using inline asm
> to prefetch read data. This was x86 specific in generic code,
> so was a little hackish..
> Now that we have the prefetch macros, I decided to play with
> this a little tonight, and came up with this patch..

Well I don't know diddly about the prefetch instruction, but.

> ...
> +
> + if (size > 128) {
> + int i;
> + for(i=0; i<size; i+=64) {
> + prefetch (kaddr+offset);
> + prefetch (kaddr+offset+(L1_CACHE_BYTES*2));
> + }
> + }
> +
> left = __copy_to_user(desc->buf, kaddr + offset, size);
> kunmap(page);
>

This needs to be inside ARCH_HAS_PREFETCH. Otherwise, for
architectures which do not implement prefetch, we have

for (i = 0; i < size; i += 64) {
{;}
{;}
}
and the compiler will *not* optimise this away. The compiler deliberately
leaves this construct alone because it assumes the programmer is asking for
a busywait loop.

We shouldn't add a busywait loop to file_read_actor(), yes?

<ot>
Reminds me of a version of the Microsoft C compiler back in about '85 which
"optimise" away

for ( ; ; )
;

Completely.
</ot>

Is prefetching 4k at a time optimal? Should the prefetch be embedded
in copy_*_user?

The code assumes that L1_CACHE_BYTES equates to the prefetch chunk.
Is this reasonable, or should it be abstracted out to a new PREFETCH_BYTES?

That `64' needs to be PREFETCH_BYTES * 2 or L1_CACHE_BYTES * 2, yes?

How come the code keeps prefetching the same address? Shouldn't
we be adding `i' to the address in there?

The code makes no attempt to align the prefetch address to anything.
Should it?

What happens if a prefetch spills over the end of the page and
faults?

> Comments ?

That'll do for now :)

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