[PATCH v3 0/6] RISC-V: Apply Zicboz to clear_page

[Andrew Jones]

On Mon, Jan 30, 2023 at 11:30:38AM -0700, Jeff Law wrote:
> 
> 
> On 1/30/23 05:01, Andrew Jones wrote:
> > When the Zicboz extension is available we can more rapidly zero naturally
> > aligned Zicboz block sized chunks of memory. As pages are always page
> > aligned and are larger than any Zicboz block size will be, then
> > clear_page() appears to be a good candidate for the extension. While cycle
> > count and energy consumption should also be considered, we can be pretty
> > certain that implementing clear_page() with the Zicboz extension is a win
> > by comparing the new dynamic instruction count with its current count[1].
> > Doing so we see that the new count is just over a quarter of the old count
> > (see patch4's commit message for more details).
> > 
> > For those of you who reviewed v1[2], you may be looking for the memset()
> > patches. As pointed out in v1, and a couple follow-up emails, it's not
> > clear that patching memset() is a win yet. When I get a chance to test
> > on real hardware with a comprehensive benchmark collection then I can
> > post the memset() patches separately (assuming the benchmarks show it's
> > worthwhile).
> So a note.  On the userspace side we are using cboz for clearing memory in
> memset.  While the data is intermixed with other changes, there's a very
> significant drop in stores and a host of related low level performance
> counters and a notable uptick in gcc #5 performance from spec2017 which is
> particularly sensitive to memory clearing.  We haven't seen any performance
> regressions attributable to using cboz across spec2017's integer suite.
> 
> I believe our current threshold setting is to use cboz for chunks >= 128
> bytes.

Thanks, Jeff! That's an encouraging report. I'll keep focused on
clear_page() with this series, but once I can get some numbers with
the memset patch, then I'll be happy to repost that as well.

Thanks,
drew