Re: [f2fs-dev] [PATCH v2] f2fs: another way to set large folio by remembering inode number

[Jaegeuk Kim]

On 05/28, Christoph Hellwig wrote:
> On Wed, May 27, 2026 at 03:59:35PM +0000, Jaegeuk Kim wrote:
> > F2FS merges bios before submit_bio, regardless of small or large folios,
> > since the block addresses are consecutive. So, I think IO subsystem was
> > working in full speed.
> 
> As does every other remotely modern file system.  But that merging is
> surprisingly expensive, which is why using folios gets really major
> performance improvements.
> 
> For one doing these checks to merge touch quite a few cache lines.
> Second, devices are often a lot more efficient if they see fewer SGL
> entries.  I.e. having a 1MB bio a single SGL tends to work better than
> having 256 of them.
> The same is true in the kernel code itself, both in the submission path
> (dma mapping and co), and even more so in the page cache handling
> both before submitting and in the completion path.
> 
> See Bart's patch about how long the walk of the bio_vecs in the f2fs
> completion path can take.  We had similar issues in XFS even in the
> workqueue completion path due to lack of rescheduling, and these simply
> go away when you do the folio manipulation in larger chunks (LAZY_PREEMPT
> would avoid the need to explicit rescheduling these days, but that just
> papers over the symptoms in this case).
> 

I see. That's also super helpful. Let me kick off the large folio support asap.
Thanks.