Re: [PATCH 5/6] KVM: Separate rmap_pde fromkvm_lpage_info->write_count

[Takuya Yoshikawa]

On Thu, 28 Jun 2012 20:39:55 +0300
Avi Kivity <avi@xxxxxxxxxx> wrote:

> > Note: write_count: 4 bytes, rmap_pde: 8 bytes.  So we are wasting
> > extra paddings by packing them into lpage_info.
> 
> The wastage is quite low since it's just 4 bytes per 2MB.

Yes.

> >> Why not just introduce a function to get the next rmap? Something like this:
> > 
> > I want to eliminate this kind of overheads.
> 
> I don't think the overhead is significant.  rmap walk speed is largely a
> function of cache misses IMO, and we may even be adding cache misses by
> splitting lpage_info.

Maybe.  But as far as I can see, write_count does not gain much from
being close to rmap_pde.

> But I still think it's the right thing since it simplifies the code.

After the rmap integration, we can remove
	if (likely(level == PT_PAGE_TABLE_LEVEL))
heuristics from __gfn_to_rmap().

As a bonus, the helper will become enough simple to be always inlined
which reduces some function calls.

> Maybe we should add a prefetch() on write_count do mitigate the
> overhead, if it starts showing up in profiles.

OK, I will post rmap integration work as soon as possible, but it
still needs to be synced with unrelated ppc works at some point in
the future: so please take that separately from this series.

Thanks,
	Takuya
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/