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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Wed, Sep 11, 2024 at 05:08:24PM GMT, Johannes Weiner wrote:
> 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@linux.dev>
> 
> 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.

The current callers use find_get_entries() and find_lock_entries() to
fill up the indices array which provides this guarantee.

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

I will add a comment in v2.

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

Thanks for the review.