Message-ID: <a530a614-41a3-4bd1-a2df-40ade7eca002@redhat.com>
Date: Thu, 1 Aug 2024 10:06:30 +0200
From: David Hildenbrand <david@...hat.com>
To: BiscuitOS Broiler <zhang.renze@....com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Cc: arnd@...db.de, linux-arch@...r.kernel.org, chris@...kel.net,
 jcmvbkbc@...il.com, James.Bottomley@...senPartnership.com, deller@....de,
 linux-parisc@...r.kernel.org, tsbogend@...ha.franken.de,
 rdunlap@...radead.org, bhelgaas@...gle.com, linux-mips@...r.kernel.org,
 richard.henderson@...aro.org, ink@...assic.park.msu.ru, mattst88@...il.com,
 linux-alpha@...r.kernel.org, jiaoxupo@....com, zhou.haofan@....com
Subject: Re: [PATCH v2 0/1] mm: introduce MADV_DEMOTE/MADV_PROMOTE

On 01.08.24 09:56, BiscuitOS Broiler wrote:
> Sure, here's the Scalable Tiered Memory Control (STMC)
> 
> **Background**
> 
> In the era when artificial intelligence, big data analytics, and
> machine learning have become mainstream research topics and
> application scenarios, the demand for high-capacity and high-
> bandwidth memory in computers has become increasingly important.
> The emergence of CXL (Compute Express Link) provides the
> possibility of high-capacity memory. Although CXL TYPE3 devices
> can provide large memory capacities, their access speed is lower
> than traditional DRAM due to hardware architecture limitations.
> 
> To enjoy the large capacity brought by CXL memory while minimizing
> the impact of high latency, Linux has introduced the Tiered Memory
> architecture. In the Tiered Memory architecture, CXL memory is
> treated as an independent, slower NUMA NODE, while DRAM is
> considered as a relatively faster NUMA NODE. Applications allocate
> memory from the local node, and Tiered Memory, leveraging memory
> reclamation and NUMA Balancing mechanisms, can transparently demote
> physical pages not recently accessed by user processes to the slower
> CXL NUMA NODE. However, when user processes re-access the demoted
> memory, the Tiered Memory mechanism will, based on certain logic,
> decide whether to promote the demoted physical pages back to the
> fast NUMA NODE. If the promotion is successful, the memory accessed
> by the user process will reside in DRAM; otherwise, it will reside in
> the CXL NODE. Through the Tiered Memory mechanism, Linux balances
> betweenlarge memory capacity and latency, striving to maintain an
> equilibrium for applications.
> 
> **Problem**
> Although Tiered Memory strives to balance between large capacity and
> latency, specific scenarios can lead to the following issues:
> 
>    1. In scenarios requiring massive computations, if data is heavily
>       stored in CXL slow memory and Tiered Memory cannot promptly
>       promote this memory to fast DRAM, it will significantly impact
>       program performance.
>    2. Similar to the scenario described in point 1, if Tiered Memory
>       decides to promote these physical pages to fast DRAM NODE, but
>       due to limitations in the DRAM NODE promote ratio, these physical
>       pages cannot be promoted. Consequently, the program will keep
>       running in slow memory.
>    3. After an application finishes computing on a large block of fast
>       memory, it may not immediately re-access it. Hence, this memory
>       can only wait for the memory reclamation mechanism to demote it.
>    4. Similar to the scenario described in point 3, if the demotion
>       speed is slow, these cold pages will occupy the promotion
>       resources, preventing some eligible slow pages from being
>       immediately promoted, severely affecting application efficiency.
> 
> **Solution**
> We propose the **Scalable Tiered Memory Control (STMC)** mechanism,
> which delegates the authority of promoting and demoting memory to the
> application. The principle is simple, as follows:
> 
>    1. When an application is preparing for computation, it can promote
>       the memory it needs to use or ensure the memory resides on a fast
>       NODE.
>    2. When an application will not use the memory shortly, it can
>       immediately demote the memory to slow memory, freeing up valuable
>       promotion resources.
> 
> STMC mechanism is implemented through the madvise system call, providing
> two new advice options: MADV_DEMOTE and MADV_PROMOTE. MADV_DEMOTE
> advises demote the physical memory to the node where slow memory
> resides; this advice only fails if there is no free physical memory on
> the slow memory node. MADV_PROMOTE advises retaining the physical memory
> in the fast memory; this advice only fails if there are no promotion
> slots available on the fast memory node. Benefits brought by STMC
> include:
> 
>    1. The STMC mechanism is a variant of on-demand memory management
>       designed to let applications enjoy fast memory as much as possible,
>       while actively demoting to slow memory when not in use, thus
>       freeing up promotion slots for the NODE and allowing it to run in
>       an optimized Tiered Memory environment.
>    2. The STMC mechanism better balances large capacity and latency.
> 
> **Shortcomings of STMC**
> The STMC mechanism requires the caller to manage memory demotion and
> promotion. If the memory is not promptly demoting after an promotion,
> it may cause issues similar to memory leaks
Ehm, that sounds scary. Can you elaborate what's happening here and why 
it is "similar to memory leaks"?


Can you also point out why migrate_pages() is not suitable? I would 
assume demote/promote is in essence simply migrating memory between nodes.

-- 
Cheers,

David / dhildenb

Powered by blists - more mailing lists