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From: Michal Hocko <mhocko@kernel.org>
To: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Dan Williams <dan.j.williams@intel.com>,
	Mel Gorman <mgorman@techsingularity.net>,
	Rik van Riel <riel@surriel.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dave Hansen <dave.hansen@intel.com>,
	Keith Busch <keith.busch@intel.com>,
	Fengguang Wu <fengguang.wu@intel.com>,
	"Du, Fan" <fan.du@intel.com>,
	"Huang, Ying" <ying.huang@intel.com>,
	Linux MM <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 0/10] Another Approach to Use PMEM as NUMA Node
Date: Wed, 27 Mar 2019 21:09:54 +0100	[thread overview]
Message-ID: <20190327193918.GP11927@dhcp22.suse.cz> (raw)
In-Reply-To: <c3690a19-e2a6-7db7-b146-b08aa9b22854@linux.alibaba.com>

On Wed 27-03-19 11:59:28, Yang Shi wrote:
> 
> 
> On 3/27/19 10:34 AM, Dan Williams wrote:
> > On Wed, Mar 27, 2019 at 2:01 AM Michal Hocko <mhocko@kernel.org> wrote:
> > > On Tue 26-03-19 19:58:56, Yang Shi wrote:
[...]
> > > > It is still NUMA, users still can see all the NUMA nodes.
> > > No, Linux NUMA implementation makes all numa nodes available by default
> > > and provides an API to opt-in for more fine tuning. What you are
> > > suggesting goes against that semantic and I am asking why. How is pmem
> > > NUMA node any different from any any other distant node in principle?
> > Agree. It's just another NUMA node and shouldn't be special cased.
> > Userspace policy can choose to avoid it, but typical node distance
> > preference should otherwise let the kernel fall back to it as
> > additional memory pressure relief for "near" memory.
> 
> In ideal case, yes, I agree. However, in real life world the performance is
> a concern. It is well-known that PMEM (not considering NVDIMM-F or HBM) has
> higher latency and lower bandwidth. We observed much higher latency on PMEM
> than DRAM with multi threads.

One rule of thumb is: Do not design user visible interfaces based on the
contemporary technology and its up/down sides. This will almost always
fire back.

Btw. if you keep arguing about performance without any numbers. Can you
present something specific?

> In real production environment we don't know what kind of applications would
> end up on PMEM (DRAM may be full, allocation fall back to PMEM) then have
> unexpected performance degradation. I understand to have mempolicy to choose
> to avoid it. But, there might be hundreds or thousands of applications
> running on the machine, it sounds not that feasible to me to have each
> single application set mempolicy to avoid it.

we have cpuset cgroup controller to help here.

> So, I think we still need a default allocation node mask. The default value
> may include all nodes or just DRAM nodes. But, they should be able to be
> override by user globally, not only per process basis.
> 
> Due to the performance disparity, currently our usecases treat PMEM as
> second tier memory for demoting cold page or binding to not memory access
> sensitive applications (this is the reason for inventing a new mempolicy)
> although it is a NUMA node.

If the performance sucks that badly then do not use the pmem as NUMA,
really. There are certainly other ways to export the pmem storage. Use
it as a fast swap storage. Or try to work on a swap caching mechanism
that still allows much faster access than a slow swap storage. But do
not try to pretend to abuse the NUMA interface while you are breaking
some of its long term established semantics.
-- 
Michal Hocko
SUSE Labs


  reply	other threads:[~2019-03-27 20:10 UTC|newest]

Thread overview: 58+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-23  4:44 [RFC PATCH 0/10] Another Approach to Use PMEM as NUMA Node Yang Shi
2019-03-23  4:44 ` [PATCH 01/10] mm: control memory placement by nodemask for two tier main memory Yang Shi
2019-03-23 17:21   ` Dan Williams
2019-03-25 19:28     ` Yang Shi
2019-03-25 23:18       ` Dan Williams
2019-03-25 23:36         ` Yang Shi
2019-03-25 23:42           ` Dan Williams
2019-03-23  4:44 ` [PATCH 02/10] mm: mempolicy: introduce MPOL_HYBRID policy Yang Shi
2019-03-23  4:44 ` [PATCH 03/10] mm: mempolicy: promote page to DRAM for MPOL_HYBRID Yang Shi
2019-03-23  4:44 ` [PATCH 04/10] mm: numa: promote pages to DRAM when it is accessed twice Yang Shi
2019-03-29  0:31   ` kbuild test robot
2019-03-23  4:44 ` [PATCH 05/10] mm: page_alloc: make find_next_best_node could skip DRAM node Yang Shi
2019-03-23  4:44 ` [PATCH 06/10] mm: vmscan: demote anon DRAM pages to PMEM node Yang Shi
2019-03-23  6:03   ` Zi Yan
2019-03-25 21:49     ` Yang Shi
2019-03-24 22:20   ` Keith Busch
2019-03-25 19:49     ` Yang Shi
2019-03-27  0:35       ` Keith Busch
2019-03-27  3:41         ` Yang Shi
2019-03-27 13:08           ` Keith Busch
2019-03-27 17:00             ` Zi Yan
2019-03-27 17:05               ` Dave Hansen
2019-03-27 17:48                 ` Zi Yan
2019-03-27 18:00                   ` Dave Hansen
2019-03-27 20:37                     ` Zi Yan
2019-03-27 20:42                       ` Dave Hansen
2019-03-28 21:59             ` Yang Shi
2019-03-28 22:45               ` Keith Busch
2019-03-23  4:44 ` [PATCH 07/10] mm: vmscan: add page demotion counter Yang Shi
2019-03-23  4:44 ` [PATCH 08/10] mm: numa: add page promotion counter Yang Shi
2019-03-23  4:44 ` [PATCH 09/10] doc: add description for MPOL_HYBRID mode Yang Shi
2019-03-23  4:44 ` [PATCH 10/10] doc: elaborate the PMEM allocation rule Yang Shi
2019-03-25 16:15 ` [RFC PATCH 0/10] Another Approach to Use PMEM as NUMA Node Brice Goglin
2019-03-25 16:56   ` Dan Williams
2019-03-25 17:45     ` Brice Goglin
2019-03-25 19:29       ` Dan Williams
2019-03-25 23:09         ` Brice Goglin
2019-03-25 23:37           ` Dan Williams
2019-03-26 12:19             ` Jonathan Cameron
2019-03-25 20:04   ` Yang Shi
2019-03-26 13:58 ` Michal Hocko
2019-03-26 18:33   ` Yang Shi
2019-03-26 18:37     ` Michal Hocko
2019-03-27  2:58       ` Yang Shi
2019-03-27  9:01         ` Michal Hocko
2019-03-27 17:34           ` Dan Williams
2019-03-27 18:59             ` Yang Shi
2019-03-27 20:09               ` Michal Hocko [this message]
2019-03-28  2:09                 ` Yang Shi
2019-03-28  6:58                   ` Michal Hocko
2019-03-28 18:58                     ` Yang Shi
2019-03-28 19:12                       ` Michal Hocko
2019-03-28 19:40                         ` Yang Shi
2019-03-28 20:40                           ` Michal Hocko
2019-03-28  8:21                   ` Dan Williams
2019-03-27 20:14               ` Dave Hansen
2019-03-27 20:35             ` Matthew Wilcox
2019-03-27 20:40               ` Dave Hansen

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