Re: [PATCH 4/5] mm: workingset: eviction buckets for bigmem/lowbit machines

[Vladimir Davydov]

On Tue, Jan 26, 2016 at 04:00:05PM -0500, Johannes Weiner wrote:
> For per-cgroup thrash detection, we need to store the memcg ID inside
> the radix tree cookie as well. However, on 32 bit that doesn't leave
> enough bits for the eviction timestamp to cover the necessary range of
> recently evicted pages. The radix tree entry would look like this:
> 
> [ RADIX_TREE_EXCEPTIONAL(2) | ZONEID(2) | MEMCGID(16) | EVICTION(12) ]
> 
> 12 bits means 4096 pages, means 16M worth of recently evicted pages.
> But refaults are actionable up to distances covering half of memory.
> To not miss refaults, we have to stretch out the range at the cost of
> how precisely we can tell when a page was evicted. This way we can
> shave off lower bits from the eviction timestamp until the necessary
> range is covered. E.g. grouping evictions into 1M buckets (256 pages)
> will stretch the longest representable refault distance to 4G.
> 
> This patch implements eviction buckets that are automatically sized
> according to the available bits and the necessary refault range, in
> preparation for per-cgroup thrash detection.
> 
> The maximum actionable distance is currently half of memory, but to
> support memory hotplug of up to 200% of boot-time memory, we size the
> buckets to cover double the distance. Beyond that, thrashing won't be
> detectable anymore.
> 
> During boot, the kernel will print out the exact parameters, like so:
> 
> [    0.113929] workingset: timestamp_bits=12 max_order=18 bucket_order=6
> 
> In this example, there are 12 radix entry bits available for the
> eviction timestamp, to cover a maximum distance of 2^18 pages (this is
> a 1G machine). Consequently, evictions must be grouped into buckets of
> 2^6 pages, or 256K.
> 
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Reviewed-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>

One nit below.

> +/*
> + * Eviction timestamps need to be able to cover the full range of
> + * actionable refaults. However, bits are tight in the radix tree
> + * entry, and after storing the identifier for the lruvec there might
> + * not be enough left to represent every single actionable refault. In
> + * that case, we have to sacrifice granularity for distance, and group
> + * evictions into coarser buckets by shaving off lower timestamp bits.
> + */
> +static unsigned int bucket_order;

__read_mostly?

Thanks,
Vladimir