Re: [PATCH v10 14/14] mm: multi-gen LRU: design doc

[Andrew Morton <akpm@linux-foundation.org>]

On Wed,  6 Apr 2022 21:15:26 -0600 Yu Zhao <yuzhao@google.com> wrote:

> Add a design doc.
> 
>
> ...
>
> +Design overview
> +===============
> +Objectives
> +----------
> +The design objectives are:
> +
> +* Good representation of access recency
> +* Try to profit from spatial locality
> +* Fast paths to make obvious choices
> +* Simple self-correcting heuristics
> +
> +The representation of access recency is at the core of all LRU
> +implementations. In the multi-gen LRU, each generation represents a
> +group of pages with similar access recency. Generations establish a
> +common frame of reference and therefore help make better choices,
> +e.g., between different memcgs on a computer or different computers in
> +a data center (for job scheduling).

Does MGLRU have any special treatment for used-once pages?

> +Exploiting spatial locality improves efficiency when gathering the
> +accessed bit. A rmap walk targets a single page and does not try to
> +profit from discovering a young PTE. A page table walk can sweep all
> +the young PTEs in an address space, but the address space can be too
> +large to make a profit. The key is to optimize both methods and use
> +them in combination.
> +
> +Fast paths reduce code complexity and runtime overhead. Unmapped pages
> +do not require TLB flushes; clean pages do not require writeback.
> +These facts are only helpful when other conditions, e.g., access
> +recency, are similar. With generations as a common frame of reference,
> +additional factors stand out. But obvious choices might not be good
> +choices; thus self-correction is required.
> +
> +The benefits of simple self-correcting heuristics are self-evident.
> +Again, with generations as a common frame of reference, this becomes
> +attainable. Specifically, pages in the same generation can be
> +categorized based on additional factors, and a feedback loop can
> +statistically compare the refault percentages across those categories
> +and infer which of them are better choices.
> +
> +Assumptions
> +-----------
> +The protection of hot pages and the selection of cold pages are based
> +on page access channels and patterns. There are two access channels:
> +
> +* Accesses through page tables
> +* Accesses through file descriptors
> +
> +The protection of the former channel is by design stronger because:
> +
> +1. The uncertainty in determining the access patterns of the former
> +   channel is higher due to the approximation of the accessed bit.
> +2. The cost of evicting the former channel is higher due to the TLB
> +   flushes required and the likelihood of encountering the dirty bit.
> +3. The penalty of underprotecting the former channel is higher because
> +   applications usually do not prepare themselves for major page
> +   faults like they do for blocked I/O. E.g., GUI applications
> +   commonly use dedicated I/O threads to avoid blocking the rendering
> +   threads.
> +
> +There are also two access patterns:
> +
> +* Accesses exhibiting temporal locality
> +* Accesses not exhibiting temporal locality
> +
> +For the reasons listed above, the former channel is assumed to follow
> +the former pattern unless ``VM_SEQ_READ`` or ``VM_RAND_READ`` is
> +present, and the latter channel is assumed to follow the latter
> +pattern unless outlying refaults have been observed.

What about MADV_SEQUENTIAL?    Or did we propogate that into the fd?

> +Workflow overview
> +=================
> +Evictable pages are divided into multiple generations for each
> +``lruvec``. The youngest generation number is stored in
> +``lrugen->max_seq`` for both anon and file types as they are aged on
> +an equal footing. The oldest generation numbers are stored in
> +``lrugen->min_seq[]`` separately for anon and file types as clean file
> +pages can be evicted regardless of swap constraints. These three
> +variables are monotonically increasing.
> +
>
> ...
>
> +Summary
> +-------
> +The multi-gen LRU can be disassembled into the following parts:
> +
> +* Generations
> +* Page table walks
> +* Rmap walks
> +* Bloom filters
> +* The PID controller
> +
> +The aging and the eviction is a producer-consumer model; specifically,
> +the latter drives the former by the sliding window over generations.
> +Within the aging, rmap walks drive page table walks by inserting hot
> +densely populated page tables to the Bloom filters. Within the
> +eviction, the PID controller uses refaults as the feedback to select
> +types to evict and tiers to protect.

It's cool to see a PID controller in there.

How do we know that it converges well, that it doesn't exhibit instability, etc?