Re: [dm-devel] [patch 4/4] dm-writecache: use new API for flushing

From: Jeff Moyer <jmoyer@redhat.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	device-mapper development <dm-devel@redhat.com>,
	Mikulas Patocka <mpatocka@redhat.com>,
	Mike Snitzer <snitzer@redhat.com>,
	linux-nvdimm <linux-nvdimm@lists.01.org>
Subject: Re: [dm-devel] [patch 4/4] dm-writecache: use new API for flushing
Date: Wed, 30 May 2018 09:42:40 -0400	[thread overview]
Message-ID: <x49h8mpmf8v.fsf@segfault.boston.devel.redhat.com> (raw)
In-Reply-To: <CAPcyv4gxCNqqG2G3R6f=kH3zNbByhgE2R-trJWurH49-p+51Gg@mail.gmail.com> (Dan Williams's message of "Mon, 28 May 2018 10:41:51 -0700")

Dan Williams <dan.j.williams@intel.com> writes:

> When I read your patch I came away with the impression that ARM had
> not added memcpy_flushcache() yet and you were working around that
> fact. Now that I look, ARM *does* define memcpy_flushcache() and
> you're avoiding it. You use memcpy+arch_wb_pmem where arch_wb_pmem on
> ARM64 is defined as __clean_dcache_area_pop(dst, cnt). The ARM
> memcpy_flushcache() implementation is:
>
>     memcpy(dst, src, cnt);
>     __clean_dcache_area_pop(dst, cnt);
>
> So, I do not see how what you're doing is any less work unless you are
> flushing less than you copy?
>
> If memcpy_flushcache() is slower than memcpy + arch_wb_pmem then the
> ARM implementation is broken and that needs to be addressed not worked
> around in a driver.

I think Mikulas wanted to batch up multiple copies and flush at the
end.  According to his commit message, that batching gained him 2%
performance.

-Jeff
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