linux-kernel - Re: [PATCH 4/7] node: Add memory caching attributes

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Date:   Mon, 19 Nov 2018 16:06:00 -0700
From:   Keith Busch <keith.busch@...el.com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-mm@...ck.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rafael Wysocki <rafael@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 4/7] node: Add memory caching attributes

On Mon, Nov 19, 2018 at 09:44:00AM +0530, Anshuman Khandual wrote:
> On 11/15/2018 04:19 AM, Keith Busch wrote:
> > System memory may have side caches to help improve access speed. While
> > the system provided cache is transparent to the software accessing
> > these memory ranges, applications can optimize their own access based
> > on cache attributes.
> 
> Cache is not a separate memory attribute. It impacts how the real attributes
> like bandwidth, latency e.g which are already captured in the previous patch.
> What is the purpose of adding this as a separate attribute ? Can you explain
> how this is going to help the user space apart from the hints it has already
> received with bandwidth, latency etc properties.

I am not sure I understand the question here. Access bandwidth and latency
are entirely attributes different than what this patch provides. If the
system side-caches memory, the associativity, line size, and total size
can optionally be used by software to improve performance.
 
> > In preparation for such systems, provide a new API for the kernel to
> > register these memory side caches under the memory node that provides it.
> 
> Under target memory node interface /sys/devices/system/node/nodeY/target* ?

Yes.
 
> > 
> > The kernel's sysfs representation is modeled from the cpu cacheinfo
> > attributes, as seen from /sys/devices/system/cpu/cpuX/cache/. Unlike CPU
> > cacheinfo, though, a higher node's memory cache level is nearer to the
> > CPU, while lower levels are closer to the backing memory. Also unlike
> > CPU cache, the system handles flushing any dirty cached memory to the
> > last level the memory on a power failure if the range is persistent.
> 
> Lets assume that a CPU has got four levels of caches L1, L2, L3, L4 before
> reaching memory. L4 is the backing cache for the memory 

I don't quite understand this question either. The cache doesn't back
the memory; the system side caches access to memory.

> and L1-L3 is from
> CPU till the system bus. Hence some of them will be represented as CPU
> caches and some of them will be represented as memory caches ?
>
> /sys/devices/system/cpu/cpuX/cache/ --> L1, L2, L3
> /sys/devices/system/node/nodeY/target --> L4 
> 
> L4 will be listed even if the node is memory only ?

The system provided memory side caches are independent of the
CPU. I'm just providing the CPU caches as a more familiar example to
compare/contrast system memory cache attributes.