Re: [PATCH 1/2] mm: optimize truncation of shadow entries

[Johannes Weiner]

On Wed, Sep 11, 2024 at 10:38:00AM -0700, Shakeel Butt wrote:
> The kernel truncates the page cache in batches of PAGEVEC_SIZE. For each
> batch, it traverses the page cache tree and collects the entries (folio
> and shadow entries) in the struct folio_batch. For the shadow entries
> present in the folio_batch, it has to traverse the page cache tree for
> each individual entry to remove them. This patch optimize this by
> removing them in a single tree traversal.
> 
> On large machines in our production which run workloads manipulating
> large amount of data, we have observed that a large amount of CPUs are
> spent on truncation of very large files (100s of GiBs file sizes). More
> specifically most of time was spent on shadow entries cleanup, so
> optimizing the shadow entries cleanup, even a little bit, has good
> impact.
> 
> To evaluate the changes, we created 200GiB file on a fuse fs and in a
> memcg. We created the shadow entries by triggering reclaim through
> memory.reclaim in that specific memcg and measure the simple truncation
> operation.
> 
>  # time truncate -s 0 file
> 
>               time (sec)
> Without       5.164 +- 0.059
> With-patch    4.21  +- 0.066 (18.47% decrease)
> 
> Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>

Looks good to me. One thing that's a bit subtle is that the tree walk
assumes indices[] are ordered, such that indices[0] and indices[nr-1]
reliably denote the range of interest. AFAICS that's the case for the
current callers but if not that could be a painful bug to hunt down.

Assessing lowest and highest index in that first batch iteration seems
a bit overkill though. Maybe just a comment stating the requirement?

Otherwise,

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>