Re: [PATCH 09/11] generic/778: fix severe performance problems

[Christoph Hellwig <hch@infradead.org>]

On Tue, Oct 21, 2025 at 11:41:33AM -0700, Darrick J. Wong wrote:
> As a result, one loop through the test takes almost 4 minutes.  The test
> loops 20 times, so it runs for 80 minutes(!!) which is a really long
> time.

Heh.  I'm glade none of my usual test setups even supports atomics I
guess :)

> So the first thing we do is observe that the giant slow loop is being
> run as a single thread on an empty filesystem.  Most of the time the
> allocator generates a mostly physically contiguous file.  We could
> fallocate the whole file instead of fallocating one block every other
> time through the loop.  This halves the setup time.
> 
> Next, we can also stuff the remaining pwrite commands into a bash array
> and only invoke xfs_io once every 128x through the loop.  This amortizes
> the xfs_io startup time, which reduces the test loop runtime to about 20
> seconds.

Wouldn't it make sense to adopt src/punch-alternating.c to also be
able to create unwritten extents instead of holes for the punched
range and run all of this from a C program?

Otherwise this looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>