From: Nilesh Choudhury <nilesh.choudhury@oracle.com>
To: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
Dan Williams <dan.j.williams@intel.com>
Cc: Jan Kara <jack@suse.cz>, Matthew Wilcox <mawilcox@microsoft.com>,
"linux-nvdimm@lists.01.org" <linux-nvdimm@ml01.01.org>,
Dave Hansen <dave.hansen@linux.intel.com>,
Linux MM <linux-mm@kvack.org>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Jan Kara <jack@suse.com>,
Andrew Morton <akpm@linux-foundation.org>,
Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH 0/3] 1G transparent hugepage support for device dax
Date: Tue, 24 Jan 2017 13:35:55 -0800 [thread overview]
Message-ID: <db42a11a-77ca-2caf-a13a-fb404d0ad2a1@oracle.com> (raw)
In-Reply-To: <20170124212435.GA23874@char.us.oracle.com>
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Konrad's explanation is precise.
There are applications which have a process model; and if you assume
10,000 processes attempting to mmap all the 6TB memory available on a
server; we are looking at the following:
processes ; 10,000
memory : 6TB
pte @ 4k page size: 8 bytes / 4K of memory * #processes = 6TB / 4k * 8 * 10000 = 1.5GB * 80000 = 120,000GB
pmd @ 2M page size: 120,000 / 512 = ~240GB
pud @ 1G page size: 240GB / 512 = ~480MB
As you can see with 2M pages, this system will use up an exorbitant
amount of DRAM to hold the page tables; but the 1G pages finally brings
it down to a reasonable level.
Memory sizes will keep increasing; so this number will keep increasing.
An argument can be made to convert the applications from process model
to thread model, but in the real world that may not be always practical.
Hopefully this helps explain the use case where this is valuable.
- Nilesh
On 1/24/2017 1:24 PM, Konrad Rzeszutek Wilk wrote:
> On Tue, Jan 24, 2017 at 10:26:54AM -0800, Dan Williams wrote:
>> On Tue, Jan 24, 2017 at 3:12 AM, Jan Kara <jack@suse.cz> wrote:
>>> On Mon 23-01-17 16:47:18, Dave Jiang wrote:
>>>> The following series implements support for 1G trasparent hugepage on
>>>> x86 for device dax. The bulk of the code was written by Mathew Wilcox
>>>> a while back supporting transparent 1G hugepage for fs DAX. I have
>>>> forward ported the relevant bits to 4.10-rc. The current submission has
>>>> only the necessary code to support device DAX.
>>> Well, you should really explain why do we want this functionality... Is
>>> anybody going to use it? Why would he want to and what will he gain by
>>> doing so? Because so far I haven't heard of a convincing usecase.
>>>
>> So the motivation and intended user of this functionality mirrors the
>> motivation and users of 1GB page support in hugetlbfs. Given expected
>> capacities of persistent memory devices an in-memory database may want
>> to reduce tlb pressure beyond what they can already achieve with 2MB
>> mappings of a device-dax file. We have customer feedback to that
>> effect as Willy mentioned in his previous version of these patches
>> [1].
> CCing Nilesh who may be able to shed some more light on this.
>
>> [1]: https://lkml.org/lkml/2016/1/31/52
>> _______________________________________________
>> Linux-nvdimm mailing list
>> Linux-nvdimm@lists.01.org
>> https://lists.01.org/mailman/listinfo/linux-nvdimm
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prev parent reply other threads:[~2017-01-24 21:35 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-23 23:47 [PATCH 0/3] 1G transparent hugepage support for device dax Dave Jiang
2017-01-23 23:47 ` Dave Jiang
2017-01-23 23:47 ` [PATCH 1/3] mm,fs,dax: Change ->pmd_fault to ->huge_fault Dave Jiang
2017-01-23 23:47 ` Dave Jiang
2017-01-23 23:47 ` [PATCH 2/3] mm, x86: Add support for PUD-sized transparent hugepages Dave Jiang
2017-01-23 23:47 ` Dave Jiang
2017-01-23 23:47 ` [PATCH 3/3] dax: Support for transparent PUD pages for device DAX Dave Jiang
2017-01-23 23:47 ` Dave Jiang
2017-01-24 11:12 ` [PATCH 0/3] 1G transparent hugepage support for device dax Jan Kara
2017-01-24 18:26 ` Dan Williams
2017-01-24 18:26 ` Dan Williams
2017-01-24 21:24 ` Konrad Rzeszutek Wilk
2017-01-24 21:35 ` Nilesh Choudhury [this message]
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