Re: [LSF/MM/BPF TOPIC] Measuring limits and enhancing buffered IO

Kent Overstreet
2024-02-27T14:57:40Z

On Tue, Feb 27, 2024 at 06:08:57AM -0800, Luis Chamberlain wrote:
> On Tue, Feb 27, 2024 at 05:07:30AM -0500, Kent Overstreet wrote:
> > On Fri, Feb 23, 2024 at 03:59:58PM -0800, Luis Chamberlain wrote:
> > > Part of the testing we have done with LBS was to do some performance
> > > tests on XFS to ensure things are not regressing. Building linux is a
> > > fine decent test and we did some random cloud instance tests on that and
> > > presented that at Plumbers, but it doesn't really cut it if we want to
> > > push things to the limit though. What are the limits to buffered IO
> > > and how do we test that? Who keeps track of it?
> > >
> > > The obvious recurring tension is that for really high performance folks
> > > just recommend to use birect IO. But if you are stress testing changes
> > > to a filesystem and want to push buffered IO to its limits it makes
> > > sense to stick to buffered IO, otherwise how else do we test it?
> > >
> > > It is good to know limits to buffered IO too because some workloads
> > > cannot use direct IO. For instance PostgreSQL doesn't have direct IO
> > > support and even as late as the end of last year we learned that adding
> > > direct IO to PostgreSQL would be difficult. Chris Mason has noted also
> > > that direct IO can also force writes during reads (?)... Anyway, testing
> > > the limits of buffered IO limits to ensure you are not creating
> > > regressions when doing some page cache surgery seems like it might be
> > > useful and a sensible thing to do .... The good news is we have not found
> > > regressions with LBS but all the testing seems to beg the question, of what
> > > are the limits of buffered IO anyway, and how does it scale? Do we know, do
> > > we care? Do we keep track of it? How does it compare to direct IO for some
> > > workloads? How big is the delta? How do we best test that? How do we
> > > automate all that? Do we want to automatically test this to avoid regressions?
> > >
> > > The obvious issues with some workloads for buffered IO is having a
> > > possible penality if you are not really re-using folios added to the
> > > page cache. Jens Axboe reported a while ago issues with workloads with
> > > random reads over a data set 10x the size of RAM and also proposed
> > > RWF_UNCACHED as a way to help [0]. As Chinner put it, this seemed more
> > > like direct IO with kernel pages and a memcpy(), and it requires
> > > further serialization to be implemented that we already do for
> > > direct IO for writes. There at least seems to be agreement that if we're
> > > going to provide an enhancement or alternative that we should strive to not
> > > make the same mistakes we've done with direct IO. The rationale for some
> > > workloads to use buffered IO is it helps reduce some tail latencies, so
> > > that's something to live up to.
> > >
> > > On that same thread Christoph also mentioned the possibility of a direct
> > > IO variant which can leverage the cache. Is that something we want to
> > > move forward with?
> > >
> > > Chris Mason also listed a few other desirables if we do:
> > >
> > > - Allowing concurrent writes (xfs DIO does this now)
> >
> > AFAIK every filesystem allows concurrent direct writes, not just xfs,
> > it's _buffered_ writes that we care about here.
>
> The context above was a possible direct IO variant, that's why direct IO
> was mentioned and that XFS at least had support.
>
> > I just pushed a patch to my CI for buffered writes without taking the
> > inode lock - for bcachefs. It'll be straightforward, but a decent amount
> > of work, to lift this to the VFS, if people are interested in
> > collaborating.
> >
> > https://evilpiepirate.org/git/bcachefs.git/log/?h=bcachefs-buffered-write-locking
>
> Neat, this is sort of what I wanted to get a sense for, if this sort of
> topic was worth discussing at LSFMM.
>
> > The approach is: for non extending, non appending writes, see if we can
> > pin the entire range of the pagecache we're writing to; fall back to
> > taking the inode lock if we can't.
>
> Perhaps a silly thought... but initial reaction is, would it make sense
> for the page cache to make this easier for us, so we have this be
> easier? It is not clear to me but my first reaction to seeing some of
> these deltas was what if we had something like the space split up, as we
> do with XFS agcounts, and so each group deals with its own ranges. I
> considered this before profiling, and as with Matthew I figured it might
> be lock contenton. It very likely is not for my test case, and as Linus
> and Dave has clarified we are both penalized and also have a
> singlthreaded writeback. If we had a group split we'd have locks per
> group and perhaps a writeback a dedicated thread per group.

Wtf are you talking about?