Re: Classification of reads within a filesystem

[Shyam Prasad N <nspmangalore@gmail.com>]

On Wed, Jul 21, 2021 at 11:22 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Jul 21, 2021 at 10:38:59PM +0530, Shyam Prasad N wrote:
> > In a scenario where a user/application issues a readahead/fadvise for
> > large data ranges in advance (informing the kernel that they intend to
> > read these data ranges soon). Depending on how much data ranges these
> > calls cover, it could keep the network quite busy for a network
> > filesystem (or the disk for a block filesystem).
> >
> > I see some value if filesystems have the ability to differentiate the
> > reads from regular buffered reads by users. In such cases, the
> > filesystem can choose to throttle the readahead reads, so that there's
> > a specified bandwidth that's still available for regular reads.
> >
> > I wanted to get your opinions about this. And whether this can be done
> > already in VFS ->readahead and ->readpage calls in the filesystems?
>
> This is something I have an interest in, but haven't had time to pursue.
> The readahead code gets this information because the page cache
> calls page_cache_sync_ra() if it needs this page right now, and calls
> page_cache_async_ra() if it thinks it will need the page in the future.
>
> ondemand_readahead() currently gets a true/false parameter
> (hit_readahead_marker), although my folio patches change it to pass in
> a folio or NULL.  That is then *not* passed to the filesystem, but it
> could be information passed in the ractl.
>


Hi Matthew,

I don't yet know if this can be useful in other scenarios.
But for the above scenario (of eagerly calling readahead), I thought
that this info can be used by a filesystem for throttling, which it
doesn't get today.
I was also thinking that there could potentially be other
classifications, apart from sync vs async, for example the process IO
priority.
Today, I don't see the process IO priority used by block layer, and
not in vfs or the individual filesystems.
Do you think this is also another info that could/should trickle down
to individual filesystems?

CCing fsdevel also to get more inputs on this.

> There's also some tidying-up to be done around faulting.  Currently
> fault-around doesn't have a way to express "read me all the pages around
> page N".  Instead it just assumes that pages N-R/2 to N+R/2 are the
> right ones to fetch when it should be left up to the filesystem or the
> readahead code to determine what window of pages to fetch.
>
> Another thing I have an interest in doing but not had opportunity to
> pursue is making ->readpage synchronous.  The current MM code always
> calls ->readahead first and only calls ->readpage if ->readahead fails.
> That means that all the async ->readpage work is actually wrong; we
> want to return the best error possible from ->readpage, even if that
> means sleeping.
>
> Oh ... except for swap.  For NFS only, it calls ->readpage, so it really
> wants ->readpage to be async so it can kick off multiple pages and
> then wait for the one it actually needs.  That gets into a conversation
> about how much we really care about swap-over-NFS, whether swap should
> be using ->readpage or ->direct_IO, and whether swap should use the
> file readahead code or its own virtual address based readahead code.
> Most of those discussions are outside my area of expertise.



-- 
Regards,
Shyam