Date:	Fri, 19 Apr 2013 13:34:57 +0800
From:	Simon Jeons <simon.jeons@...il.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
CC:	akpm@...ux-foundation.org, mgorman@...e.de,
	matthew.garrett@...ula.com, dave@...1.net, rientjes@...gle.com,
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Subject: Re: [RFC PATCH v2 00/15][Sorted-buddy] mm: Memory Power Management

Hi Srivatsa,
On 04/10/2013 05:45 AM, Srivatsa S. Bhat wrote:
> [I know, this cover letter is a little too long, but I wanted to clearly
> explain the overall goals and the high-level design of this patchset in
> detail. I hope this helps more than it annoys, and makes it easier for
> reviewers to relate to the background and the goals of this patchset.]
>
>
> Overview of Memory Power Management and its implications to the Linux MM
> ========================================================================
>
> Today, we are increasingly seeing computer systems sporting larger and larger
> amounts of RAM, in order to meet workload demands. However, memory consumes a
> significant amount of power, potentially upto more than a third of total system
> power on server systems. So naturally, memory becomes the next big target for
> power management - on embedded systems and smartphones, and all the way upto
> large server systems.
>
> Power-management capabilities in modern memory hardware:
> -------------------------------------------------------
>
> Modern memory hardware such as DDR3 support a number of power management
> capabilities - for instance, the memory controller can automatically put

memory controller is integrated in cpu in NUMA system and mount on PCI-E 
in UMA, correct? How can memory controller know which memory DIMMs/banks 
it will control?

> memory DIMMs/banks into content-preserving low-power states, if it detects
> that that *entire* memory DIMM/bank has not been referenced for a threshold
> amount of time, thus reducing the energy consumption of the memory hardware.
> We term these power-manageable chunks of memory as "Memory Regions".
>
> Exporting memory region info of the platform to the OS:
> ------------------------------------------------------
>
> The OS needs to know about the granularity at which the hardware can perform
> automatic power-management of the memory banks (i.e., the address boundaries
> of the memory regions). On ARM platforms, the bootloader can be modified to
> pass on this info to the kernel via the device-tree. On x86 platforms, the
> new ACPI 5.0 spec has added support for exporting the power-management
> capabilities of the memory hardware to the OS in a standard way[5].
>
> Estimate of power-savings from power-aware Linux MM:
> ---------------------------------------------------
>
> Once the firmware/bootloader exports the required info to the OS, it is upto
> the kernel's MM subsystem to make the best use of these capabilities and manage
> memory power-efficiently. It had been demonstrated on a Samsung Exynos board
> (with 2 GB RAM) that upto 6 percent of total system power can be saved by
> making the Linux kernel MM subsystem power-aware[4]. (More savings can be
> expected on systems with larger amounts of memory, and perhaps improved further
> using better MM designs).

How to know there are 6 percent of total system power can be saved by 
making the Linux kernel MM subsystem power-aware?

>
>
> Role of the Linux MM in enhancing memory power savings:
> ------------------------------------------------------
>
> Often, this simply translates to having the Linux MM understand the granularity
> at which RAM modules can be power-managed, and keeping the memory allocations
> and references consolidated to a minimum no. of these power-manageable
> "memory regions". It is of particular interest to note that most of these memory
> hardware have the intelligence to automatically save power, by putting memory
> banks into (content-preserving) low-power states when not referenced for a

How to know DIMM/bank is not referenced?

> threshold amount of time. All that the kernel has to do, is avoid wrecking
> the power-savings logic by scattering its allocations and references all over
> the system memory. (The kernel/MM doesn't have to perform the actual power-state
> transitions; its mostly done in the hardware automatically, and this is OK
> because these are *content-preserving* low-power states).
>
> So we can summarize the goals for the Linux MM as:
>
> o Consolidate memory allocations and/or references such that they are not
> spread across the entire memory address space.  Basically the area of memory
> that is not being referenced can reside in low power state.
>
> o Support light-weight targetted memory compaction/reclaim, to evacuate
> lightly-filled memory regions. This helps avoid memory references to
> those regions, thereby allowing them to reside in low power states.
>
>
> Assumptions and goals of this patchset:
> --------------------------------------
>
> In this patchset, we don't handle the part of getting the region boundary info
> from the firmware/bootloader and populating it in the kernel data-structures.
> The aim of this patchset is to propose and brainstorm on a power-aware design
> of the Linux MM which can *use* the region boundary info to influence the MM
> at various places such as page allocation, reclamation/compaction etc, thereby
> contributing to memory power savings. (This patchset is very much an RFC at
> the moment and is not intended for mainline-inclusion yet).
>
> So, in this patchset, we assume a simple model in which each 512MB chunk of
> memory can be independently power-managed, and hard-code this into the patchset.
> As mentioned, the focus of this patchset is not so much on how we get this info
> from the firmware or how exactly we handle a variety of configurations, but
> rather on discussing the power-savings/performance impact of the MM algorithms
> that *act* upon this info in order to save memory power.
>
> That said, its not very far-fetched to try this out with actual region
> boundary info to get the actual power savings numbers. For example, on ARM
> platforms, we can make the bootloader export this info to the OS via device-tree
> and then run this patchset. (This was the method used to get the power-numbers
> in [4]). But even without doing that, we can very well evaluate the
> effectiveness of this patchset in contributing to power-savings, by analyzing
> the free page statistics per-memory-region; and we can observe the performance
> impact by running benchmarks - this is the approach currently used to evaluate
> this patchset.
>
>
> Brief overview of the design/approach used in this patchset:
> -----------------------------------------------------------
>
> This patchset implements the 'Sorted-buddy design' for Memory Power Management,
> in which the buddy (page) allocator is altered to keep the buddy freelists
> region-sorted, which helps influence the page allocation paths to keep the

If this will impact normal zone based buddy freelists?

> allocations consolidated to a minimum no. of memory regions. This patchset also
> includes a light-weight targetted compaction/reclaim algorithm that works
> hand-in-hand with the page-allocator, to evacuate lightly-filled memory regions
> when memory gets fragmented, in order to further enhance memory power savings.
>
> This Sorted-buddy design was developed based on some of the suggestions
> received[1] during the review of the earlier patchset on Memory Power
> Management written by Ankita Garg ('Hierarchy design')[2].
> One of the key aspects of this Sorted-buddy design is that it avoids the
> zone-fragmentation problem that was present in the earlier design[3].
>
>
>
> Design of sorted buddy allocator and light-weight targetted region compaction:
> =============================================================================
>
> Sorted buddy allocator:
> ----------------------
>
> In this design, the memory region boundaries are captured in a data structure
> parallel to zones, instead of fitting regions between nodes and zones in the
> hierarchy. Further, the buddy allocator is altered, such that we maintain the
> zones' freelists in region-sorted-order and thus do page allocation in the
> order of increasing memory regions. (The freelists need not be fully
> address-sorted, they just need to be region-sorted).
>
> The idea is to do page allocation in increasing order of memory regions
> (within a zone) and perform region-compaction in the reverse order, as
> illustrated below.
>
> ---------------------------- Increasing region number---------------------->
>
> Direction of allocation--->               <---Direction of region-compaction
>
>
> The sorting logic (to maintain freelist pageblocks in region-sorted-order)
> lies in the page-free path and hence the critical page-allocation paths remain
> fast. Also, the sorting logic is optimized to be O(log n).
>
> Advantages of this design:
> --------------------------
> 1. No zone-fragmentation (IOW, we don't create more zones than necessary) and
>     hence we avoid its associated problems (like too many zones, extra kswapd
>     activity, question of choosing watermarks etc).
>     [This is an advantage over the 'Hierarchy' design]
>
> 2. Performance overhead is expected to be low: Since we retain the simplicity
>     of the algorithm in the page allocation path, page allocation can
>     potentially remain as fast as it would be without memory regions. The
>     overhead is pushed to the page-freeing paths which are not that critical.
>
>
> Light-weight targetted region compaction:
> ----------------------------------------
>
> Over time, due to multiple alloc()s and free()s in random order, memory gets
> fragmented, which means the memory allocations will no longer be consolidated
> to a minimum no. of memory regions. In such cases we need a light-weight
> mechanism to opportunistically compact memory to evacuate lightly-filled
> memory regions, thereby enhancing the power-savings.
>
> Noting that CMA (Contiguous Memory Allocator) does targetted compaction to
> achieve its goals, this patchset generalizes the targetted compaction code
> and reuses it to evacuate memory regions. The region evacuation is triggered
> by the page allocator : when it notices the first page allocation in a new
> region, it sets up a worker function to perform compaction and evacuate that
> region in the future, if possible. There are handshakes between the alloc
> and the free paths in the page allocator to help do this smartly, which are
> explained in detail in the patches.
>
>
> This patchset has been hosted in the below git tree. It applies cleanly on
> v3.9-rc5.
>
> git://github.com/srivatsabhat/linux.git mem-power-mgmt-v2
>
>
> Changes in this v2:
> ==================
>
> * Fixed a bug in the NUMA case.
> * Added a new optimized O(log n) sorting algorithm to speed up region-sorting
>    of the buddy freelists (patch 9). The efficiency of this new algorithm and
>    its design allows us to support large amounts of RAM quite easily.
> * Added light-weight targetted compaction/reclaim support for memory power
>    management (patches 10-14).
> * Revamped the cover-letter to better explain the idea behind memory power
>    management and this patchset.
>
>
> Experimental Results:
> ====================
>
> Test setup:
> ----------
>
> x86 dual-socket quad core HT-enabled machine booted with mem=8G
> Memory region size = 512 MB
>
> Functional testing:
> ------------------
>
> Ran pagetest, a simple C program that allocates and touches a required number
> of pages.
>
> Below is the statistics from the regions within ZONE_NORMAL, at various sizes
> of allocations from pagetest.
>
>
> 	     Present pages   |	Free pages at various allocation sizes   |
> 			     |  start	|  512 MB  |  1024 MB | 2048 MB  |
>    Region 0           1	     |      0   |      0   |       0  |       0  |
>    Region 1      131072       |  41537   |  13858   |   13790  |   13334  |
>    Region 2      131072       | 131072   |  26839   |      82  |     122  |
>    Region 3      131072       | 131072   | 131072   |   26624  |       0  |
>    Region 4      131072       | 131072   | 131072   |  131072  |       0  |
>    Region 5      131072       | 131072   | 131072   |  131072  |   26624  |
>    Region 6      131072       | 131072   | 131072   |  131072  |  131072  |
>    Region 7      131072       | 131072   | 131072   |  131072  |  131072  |
>    Region 8      131071       |  72704   |  72704   |   72704  |   72704  |
>
> This shows that page allocation occurs in the order of increasing region
> numbers, as intended in this design.
>
> Performance impact:
> -------------------
>
> Kernbench results didn't show any noticeable performance degradation with
> this patchset as compared to vanilla 3.9-rc5.
>
>
> Todos and ideas for enhancing the design further:
> ================================================
>
> 1. Add support for making this work with sparsemem, memcg etc.
>
> 2. Mel Gorman pointed out that regular compaction algorithm would work
>     against the sorted-buddy allocation strategy, since it creates free space
>     at lower pfns. For now, I have not handled this because regular compaction
>     triggers only when the memory pressure is very high, and hence memory
>     power management is pointless in those situations. Besides, it is
>     immaterial whether memory allocations are consolidated towards lower or
>     higher pfns, because it saves power either way, and hence the regular
>     compaction algorithm doesn't actually work against memory power management.
>
> 3. Add more optimizations to the targetted region compaction algorithm in order
>     to enhance its benefits and reduce the overhead, such as:
>     a. Migrate only active pages during region evacuation, because, strictly
>        speaking we only want to avoid _references_ to the region. So inactive
>        pages can be kept around, thus reducing the page-migration overhead.
>     b. Reduce the search-space for region evacuation, by having the
>        page-allocator note down the highest allocated pfn within that region.
>
> 4. Have stronger influence over how freepages from different migratetypes
>     are exchanged, so that unmovable and non-reclaimable allocations are
>     contained within least no. of memory regions.
>
> 5. Influence the refill of per-cpu pagesets and perhaps even heavily used
>     slab caches, such that they all get their memory from least no. of memory
>     regions. This is to avoid frequent fragmentation of memory regions.
>
> 6. Don't perform region evacuation at situations of high memory utilization.
>     Also, never use freepages from MIGRATE_RESERVE for the purpose of
>     region-evacuation.
>
> 7. Add more tracing/debug info to enable better evaluation of the
>     effectiveness and benefits of this patchset over vanilla kernel.
>
> 8. Add a higher level policy to control the aggressiveness of memory power
>     management.
>
>
> References:
> ----------
>
> [1]. Review comments suggesting modifying the buddy allocator to be aware of
>       memory regions:
>       http://article.gmane.org/gmane.linux.power-management.general/24862
>       http://article.gmane.org/gmane.linux.power-management.general/25061
>       http://article.gmane.org/gmane.linux.kernel.mm/64689
>
> [2]. Patch series that implemented the node-region-zone hierarchy design:
>       http://lwn.net/Articles/445045/
>       http://thread.gmane.org/gmane.linux.kernel.mm/63840
>
>       Summary of the discussion on that patchset:
>       http://article.gmane.org/gmane.linux.power-management.general/25061
>
>       Forward-port of that patchset to 3.7-rc3 (minimal x86 config)
>       http://thread.gmane.org/gmane.linux.kernel.mm/89202
>
> [3]. Disadvantages of having memory regions in the hierarchy between nodes and
>       zones:
>       http://article.gmane.org/gmane.linux.kernel.mm/63849
>
> [4]. Estimate of potential power savings on Samsung exynos board
>       http://article.gmane.org/gmane.linux.kernel.mm/65935
>
> [5]. ACPI 5.0 and MPST support
>       http://www.acpi.info/spec.htm
>       Section 5.2.21 Memory Power State Table (MPST)
>
> [6]. v1 of Sorted-buddy memory power management patchset:
>       http://thread.gmane.org/gmane.linux.power-management.general/28498
>
>
>   Srivatsa S. Bhat (15):
>        mm: Introduce memory regions data-structure to capture region boundaries within nodes
>        mm: Initialize node memory regions during boot
>        mm: Introduce and initialize zone memory regions
>        mm: Add helpers to retrieve node region and zone region for a given page
>        mm: Add data-structures to describe memory regions within the zones' freelists
>        mm: Demarcate and maintain pageblocks in region-order in the zones' freelists
>        mm: Add an optimized version of del_from_freelist to keep page allocation fast
>        bitops: Document the difference in indexing between fls() and __fls()
>        mm: A new optimized O(log n) sorting algo to speed up buddy-sorting
>        mm: Add support to accurately track per-memory-region allocation
>        mm: Restructure the compaction part of CMA for wider use
>        mm: Add infrastructure to evacuate memory regions using compaction
>        mm: Implement the worker function for memory region compaction
>        mm: Add alloc-free handshake to trigger memory region compaction
>        mm: Print memory region statistics to understand the buddy allocator behavior
>
>
>    arch/x86/include/asm/bitops.h      |    4
>   include/asm-generic/bitops/__fls.h |    5
>   include/linux/compaction.h         |    7
>   include/linux/gfp.h                |    2
>   include/linux/migrate.h            |    3
>   include/linux/mm.h                 |   62 ++++
>   include/linux/mmzone.h             |   78 ++++-
>   include/trace/events/migrate.h     |    3
>   mm/compaction.c                    |  149 +++++++++
>   mm/internal.h                      |   40 ++
>   mm/page_alloc.c                    |  617 ++++++++++++++++++++++++++++++++----
>   mm/vmstat.c                        |   36 ++
>   12 files changed, 935 insertions(+), 71 deletions(-)
>
>
> Regards,
> Srivatsa S. Bhat
> IBM Linux Technology Center
>
> --
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