Re: [PATCH v3 2/2] zswap: track swapins from disk more accurately

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Aug 05, 2024 at 04:22:43PM -0700, Nhat Pham wrote:
> Currently, there are a couple of issues with our disk swapin tracking
> for dynamic zswap shrinker heuristics:
> 
> 1. We only increment the swapin counter on pivot pages. This means we
>    are not taking into account pages that also need to be swapped in,
>    but are already taken care of as part of the readahead window.
> 
> 2. We are also incrementing when the pages are read from the zswap pool,
>    which is inaccurate.
> 
> This patch rectifies these issues by incrementing the counter whenever
> we need to perform a non-zswap read. Note that we are slightly
> overcounting, as a page might be read into memory by the readahead
> algorithm even though it will not be neeeded by users - however, this is
> an acceptable inaccuracy, as the readahead logic itself will adapt to
> these kind of scenarios.
> 
> To test this change, I built the kernel under a cgroup with its
> memory.max set to 2 GB:
> 
> real: 236.66s
> user: 4286.06s
> sys: 652.86s
> swapins: 81552
> 
> For comparison, with just the new second chance algorithm, the build
> time is as follows:
> 
> real: 244.85s
> user: 4327.22s
> sys: 664.39s
> swapins: 94663
> 
> Without neither:
> 
> real: 263.89s
> user: 4318.11s
> sys: 673.29s
> swapins: 227300.5
> 
> (average over 5 runs)
> 
> With this change, the kernel CPU time reduces by a further 1.7%, and
> the real time is reduced by another 3.3%, compared to just the second
> chance algorithm by itself. The swapins count also reduces by another
> 13.85%.
> 
> Combinng the two changes, we reduce the real time by 10.32%, kernel CPU
> time by 3%, and number of swapins by 64.12%.
> 
> To gauge the new scheme's ability to offload cold data, I ran another
> benchmark, in which the kernel was built under a cgroup with memory.max
> set to 3 GB, but with 0.5 GB worth of cold data allocated before each
> build (in a shmem file).
> 
> Under the old scheme:
> 
> real: 197.18s
> user: 4365.08s
> sys: 289.02s
> zswpwb: 72115.2
> 
> Under the new scheme:
> 
> real: 195.8s
> user: 4362.25s
> sys: 290.14s
> zswpwb: 87277.8
> 
> (average over 5 runs)
> 
> Notice that we actually observe a 21% increase in the number of written
> back pages - so the new scheme is just as good, if not better at
> offloading pages from the zswap pool when they are cold. Build time
> reduces by around 0.7% as a result.
> 
> Fixes: b5ba474f3f51 ("zswap: shrink zswap pool based on memory pressure")
> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Nhat Pham <nphamcs@gmail.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>