Re: [RFC PATCH v1 0/4] Control folio sizes used for page cache memory

[David Hildenbrand <david@redhat.com>]

On 17.07.24 09:12, Ryan Roberts wrote:
> Hi All,
> 
> This series is an RFC that adds sysfs and kernel cmdline controls to configure
> the set of allowed large folio sizes that can be used when allocating
> file-memory for the page cache. As part of the control mechanism, it provides
> for a special-case "preferred folio size for executable mappings" marker.
> 
> I'm trying to solve 2 separate problems with this series:
> 
> 1. Reduce pressure in iTLB and improve performance on arm64: This is a modified
> approach for the change at [1]. Instead of hardcoding the preferred executable
> folio size into the arch, user space can now select it. This decouples the arch
> code and also makes the mechanism more generic; it can be bypassed (the default)
> or any folio size can be set. For my use case, 64K is preferred, but I've also
> heard from Willy of a use case where putting all text into 2M PMD-sized folios
> is preferred. This approach avoids the need for synchonous MADV_COLLAPSE (and
> therefore faulting in all text ahead of time) to achieve that.
> 
> 2. Reduce memory fragmentation in systems under high memory pressure (e.g.
> Android): The theory goes that if all folios are 64K, then failure to allocate a
> 64K folio should become unlikely. But if the page cache is allocating lots of
> different orders, with most allocations having an order below 64K (as is the
> case today) then ability to allocate 64K folios diminishes. By providing control
> over the allowed set of folio sizes, we can tune to avoid crucial 64K folio
> allocation failure. Additionally I've heard (second hand) of the need to disable
> large folios in the page cache entirely due to latency concerns in some
> settings. These controls allow all of this without kernel changes.
> 
> The value of (1) is clear and the performance improvements are documented in
> patch 2. I don't yet have any data demonstrating the theory for (2) since I
> can't reproduce the setup that Barry had at [2]. But my view is that by adding
> these controls we will enable the community to explore further, in the same way
> that the anon mTHP controls helped harden the understanding for anonymous
> memory.
> 
> ---

How would this interact with other requirements we get from the 
filesystem (for example, because of the device) [1].

Assuming a device has a filesystem has a min order of X, but we disable 
anything >= X, how would we combine that configuration/information?


[1] 
https://lore.kernel.org/all/20240715094457.452836-2-kernel@pankajraghav.com/T/#u

-- 
Cheers,

David / dhildenb