On 17.07.24 12:45, Ryan Roberts wrote:
On 17/07/2024 11:31, David Hildenbrand wrote:
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?
Currently order-0 is implicitly the "always-on" fallback order. My thinking was
that with [1], the specified min order just becomes that "always-on" fallback order.
Today:
orders = file_orders_always() | BIT(0);
Tomorrow:
orders = (file_orders_always() & ~(BIT(min_order) - 1)) | BIT(min_order);
That does mean that in this case, a user-disabled order could still be used. So
the controls are really hints rather than definitive commands.
Okay, because that's a difference to order-0, which is -- as you note --
always-on (not even a toggle).
Staring at patch #1, you use the name "file_enable". That might indeed
cause some confusion. Thinking out loud, I wonder if a different
terminology could better express the semantics. Hm ... but maybe it only
would have to be documented.
Thanks for the details.
--
Cheers,
David / dhildenb
