From: Matthew Wilcox <willy@linux.intel.com>
To: Rik van Riel <riel@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Dan Williams <dan.j.williams@intel.com>,
linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
axboe@kernel.dk, linux-nvdimm@ml01.01.org,
Dave Hansen <dave.hansen@linux.intel.com>,
linux-raid@vger.kernel.org, mgorman@suse.de, hch@infradead.org,
linux-fsdevel@vger.kernel.org,
"Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [RFC PATCH 0/7] evacuate struct page from the block layer
Date: Fri, 20 Mar 2015 16:31:36 -0400 [thread overview]
Message-ID: <20150320203136.GM4003@linux.intel.com> (raw)
In-Reply-To: <550C490E.1080708@redhat.com>
On Fri, Mar 20, 2015 at 12:21:34PM -0400, Rik van Riel wrote:
> On 03/19/2015 09:43 AM, Matthew Wilcox wrote:
>
> > 1. Construct struct pages for persistent memory
> > 1a. Permanently
> > 1b. While the pages are under I/O
>
> Michael Tsirkin and I have been doing some thinking about what
> it would take to allocate struct pages per 2MB area permanently,
> and allocate additional struct pages for 4kB pages on demand,
> when a 2MB area is broken up into 4kB pages.
Ah! I've looked at that a couple of times as well. I asked our database
performance team what impact freeing up the memmap would have on their
performance. They told me that doubling the amount of memory generally
resulted in approximately a 40% performance improvement. So freeing up
1.5% additional memory would result in about 0.6% performance improvement,
which I thought was probably too small a return on investment to justify
turning memmap into a two-level data structure.
Persistent memory might change that calculation somewhat ... but I'm
not convinced. Certainly, if we already had the ability to allocate
'struct superpage', I wouldn't be pushing for page-less I/Os, I'd just
allocate these data structures for PM. Even if they were 128 bytes in
size, that's only a 25MB overhead per 400GB NV-DIMM, which feels quite
reasonable to me.
> This should work for both DRAM and persistent memory.
>
> I am still not convinced it is worthwhile to have struct pages
> for persistent memory though, but I am willing to change my mind.
There's a lot of code out there that relies on struct page being PAGE_SIZE
bytes. I'm cool with replacing 'struct page' with 'struct superpage'
[1] in the biovec and auditing all of the code which touches it ... but
that's going to be a lot of code! I'm not sure it's less code than
going directly to 'just do I/O on PFNs'.
[1] Please, somebody come up with a better name!
next prev parent reply other threads:[~2015-03-20 20:31 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-03-16 20:25 [RFC PATCH 0/7] evacuate struct page from the block layer Dan Williams
2015-03-16 20:25 ` [RFC PATCH 1/7] block: add helpers for accessing a bio_vec page Dan Williams
2015-03-16 20:25 ` [RFC PATCH 2/7] block: convert bio_vec.bv_page to bv_pfn Dan Williams
2015-03-16 23:05 ` Al Viro
2015-03-17 13:02 ` Matthew Wilcox
2015-03-17 15:53 ` Dan Williams
2015-03-16 20:25 ` [RFC PATCH 3/7] dma-mapping: allow archs to optionally specify a ->map_pfn() operation Dan Williams
2015-03-18 11:21 ` [Linux-nvdimm] " Boaz Harrosh
2015-03-16 20:25 ` [RFC PATCH 4/7] scatterlist: use sg_phys() Dan Williams
2015-03-16 20:25 ` [RFC PATCH 5/7] scatterlist: support "page-less" (__pfn_t only) entries Dan Williams
2015-03-16 20:25 ` [RFC PATCH 6/7] x86: support dma_map_pfn() Dan Williams
2015-03-16 20:26 ` [RFC PATCH 7/7] block: base support for pfn i/o Dan Williams
2015-03-18 10:47 ` [RFC PATCH 0/7] evacuate struct page from the block layer Boaz Harrosh
2015-03-18 13:06 ` Matthew Wilcox
2015-03-18 14:38 ` [Linux-nvdimm] " Boaz Harrosh
2015-03-20 15:56 ` Rik van Riel
2015-03-22 11:53 ` Boaz Harrosh
2015-03-18 15:35 ` Dan Williams
2015-03-18 20:26 ` Andrew Morton
2015-03-19 13:43 ` Matthew Wilcox
2015-03-19 15:54 ` [Linux-nvdimm] " Boaz Harrosh
2015-03-19 19:59 ` Andrew Morton
2015-03-19 20:59 ` Dan Williams
2015-03-22 17:22 ` Boaz Harrosh
2015-03-20 17:32 ` Wols Lists
2015-03-22 10:30 ` Boaz Harrosh
2015-03-19 18:17 ` Christoph Hellwig
2015-03-19 19:31 ` Matthew Wilcox
2015-03-22 16:46 ` Boaz Harrosh
2015-03-20 16:21 ` Rik van Riel
2015-03-20 20:31 ` Matthew Wilcox [this message]
2015-03-20 21:08 ` Rik van Riel
2015-03-22 17:06 ` Boaz Harrosh
2015-03-22 17:22 ` Dan Williams
2015-03-22 17:39 ` Boaz Harrosh
2015-03-20 21:17 ` Wols Lists
2015-03-22 16:24 ` Boaz Harrosh
2015-03-22 15:51 ` Boaz Harrosh
2015-03-23 15:19 ` Rik van Riel
2015-03-23 19:30 ` Christoph Hellwig
2015-03-24 9:41 ` Boaz Harrosh
2015-03-24 16:57 ` Rik van Riel
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