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Re: [Cluster-devel] [RFC PATCH 0/2] dirreadahead system call



On Thu, Jul 31, 2014 at 01:19:45PM +0200, Andreas Dilger wrote:
> On Jul 31, 2014, at 6:49, Dave Chinner <david fromorbit com> wrote:
> > 
> >> On Mon, Jul 28, 2014 at 03:19:31PM -0600, Andreas Dilger wrote:
> >>> On Jul 28, 2014, at 6:52 AM, Abhijith Das <adas redhat com> wrote:
> >>> OnJuly 26, 2014 12:27:19 AM "Andreas Dilger" <adilger dilger ca> wrote:
> >>>> Is there a time when this doesn't get called to prefetch entries in
> >>>> readdir() order?  It isn't clear to me what benefit there is of returning
> >>>> the entries to userspace instead of just doing the statahead implicitly
> >>>> in the kernel?
> >>>> 
> >>>> The Lustre client has had what we call "statahead" for a while,
> >>>> and similar to regular file readahead it detects the sequential access
> >>>> pattern for readdir() + stat() in readdir() order (taking into account if
> >>>> ".*"
> >>>> entries are being processed or not) and starts fetching the inode
> >>>> attributes asynchronously with a worker thread.
> >>> 
> >>> Does this heuristic work well in practice? In the use case we were trying to
> >>> address, a Samba server is aware beforehand if it is going to stat all the
> >>> inodes in a directory.
> >> 
> >> Typically this works well for us, because this is done by the Lustre
> >> client, so the statahead is hiding the network latency of the RPCs to
> >> fetch attributes from the server.  I imagine the same could be seen with
> >> GFS2. I don't know if this approach would help very much for local
> >> filesystems because the latency is low.
> >> 
> >>>> This syscall might be more useful if userspace called readdir() to get
> >>>> the dirents and then passed the kernel the list of inode numbers
> >>>> to prefetch before starting on the stat() calls. That way, userspace
> >>>> could generate an arbitrary list of inodes (e.g. names matching a
> >>>> regexp) and the kernel doesn't need to guess if every inode is needed.
> >>> 
> >>> Were you thinking arbitrary inodes across the filesystem or just a subset
> >>> from a directory? Arbitrary inodes may potentially throw up locking issues.
> >> 
> >> I was thinking about inodes returned from readdir(), but the syscall
> >> would be much more useful if it could handle arbitrary inodes.
> > 
> > I'm not sure we can do that. The only way to safely identify a
> > specific inode in the filesystem from userspace is via a filehandle.
> > Plain inode numbers are susceptible to TOCTOU race conditions that
> > the kernel cannot resolve. Also, lookup by inode number bypasses
> > directory access permissions, so is not something we would expose
> > to arbitrary unprivileged users.
> 
> None of these issues are relevant in the API that I'm thinking about. 
> The syscall just passes the list of inode numbers to be prefetched
> into kernel memory, and then stat() is used to actually get the data into
> userspace (or whatever other operation is to be done on them),
> so there is no danger if the wrong inode is prefetched.  If the inode
> number is bad the filesystem can just ignore it. 

Which means the filesystem has to treat the inode number as
potentially hostile. i.e. it can not be trusted to be correct and so
must take slow paths to validate the inode numbers. This adds
*significant* overhead to the readahead path for some filesystems:
readahead is only a win if it is low cost.

For example, on XFS every untrusted inode number lookup requires an
inode btree lookup to validate the inode is actually valid on disk
and that is it allocated and has references. That lookup serialises
against inode allocation/freeing as well as other lookups. In
comparison, when using a trusted inode number from a directory
lookup within the kernel, we only need to do a couple of shift and
mask operations to convert it to a disk address and we are good to
go.

i.e. the difference is at least 5 orders of magnitude higher CPU usage
for an "inode number readahead" syscall versus a "directory
readahead" syscall, it has significant serialisation issues and it
can stall other modification/lookups going on at the same time.
That's *horrible behaviour* for a speculative readahead operation,
but because the inodenumbers are untrusted, we can't avoid it.

So, again, it's way more overhead than userspace just calling
stat() asycnhronously on many files at once as readdir/gentdents
returns dirents from the kernel to speed up cache population.

That's my main issue with this patchset - it's implementing
something in kernelspace that can *easily* be done generically in
userspace without introducing all sorts of nasty corner cases that
we have to handle in the kernel. We only add functionality to the kernel if there's a
compelling reason to do it in kernelspace, and right now I just
don't see any numbers that justify adding readdir+stat() readahead
or inode number based cache population in kernelspace.

Before we add *any* syscall for directory readahead, we need
comparison numbers against doing the dumb multithreaded
userspace readahead of stat() calls. If userspace can do this as
fast as the kernel can....

Cheers,

Dave.
-- 
Dave Chinner
david fromorbit com


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