Re: I can't get no readdir satisfaction

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On 24 Aug 2016, at 10:19, Trond Myklebust wrote:

On Aug 24, 2016, at 10:16, Benjamin Coddington <bcodding@xxxxxxxxxx> wrote:

On 24 Aug 2016, at 10:02, Trond Myklebust wrote:

On Aug 24, 2016, at 09:56, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote:

On Wed, Aug 24, 2016 at 12:18:04PM +0000, Trond Myklebust wrote:

On Aug 23, 2016, at 17:21, Benjamin Coddington <bcodding@xxxxxxxxxx> wrote:



On 23 Aug 2016, at 11:36, Trond Myklebust wrote:

On Aug 23, 2016, at 11:09, Benjamin Coddington <bcodding@xxxxxxxxxx> wrote:

Hi linux-nfs,

311324ad1713 ("NFS: Be more aggressive in using readdirplus for 'ls -l' situations") changed when nfs_readdir() decides to revalidate the directory's mapping, which contains all the entries. In addition to just checking if the attribute cache has expired, it includes a check to see if
NFS_INO_INVALID_DATA is set on the directory.

Well, customers that have directories with very many dentries and that same directory's attributes are frequently updated are now grouchy that `ls -l` takes so long since any update of the directory causes the mapping to be invalidated and we have to start over filling the directory's mapping.

I actually haven't put real hard thought into it yet (because often for me that just wastes a lot of time), so I am doing the lazy thing by asking this
question:

Can we go back to just the using the attribute cache timeout, or should we
get all heuristical about readdir?


We are all heuristical at this point. How are the heuristics failing?

The original problem those heuristics were designed to solve was that all the stat() calls took forever to complete, since they are all synchronous; Tigran showed some very convincing numbers for a large directory where the difference in performance was an order of magnitude improved by using
readdirplus instead of readdir…

I'll try to present a better explanation. While `ls -l` is walking through a directory repeatedly entering nfs_readdir(), a CREATE response send us
through nfs_post_op_update_inode_locked():

1531     if (S_ISDIR(inode->i_mode))
1532         invalid |= NFS_INO_INVALID_DATA;
1533     nfs_set_cache_invalid(inode, invalid);

Now, the next entry into nfs_readdir() has us do nfs_revalidate_mapping(), which will do nfs_invalidate_mapping() for the directory, and so we have to start over with cookie 0 sending READDIRPLUS to re-fill the directory's mapping to get back to where we are for the current nfs_readdir().

This process repeats for every entry into nfs_readdir() if the directory keeps getting updated, and it becomes more likely that it will be updated as each pass takes longer and longer to re-fill the mapping as the current
nfs_readdir() invocation is further along.

READDIRPLUS isn't the problem, the problem is invalidating the directory mapping in the middle of a series of getdents() if we do a CREATE. Also, I think a similar thing happens if the directory's mtime or ctime is updated - but in that case we set NFS_INO_INVALID_DATA because the change_attr
updates.

So, for directories with a large number of entries that updates often, it can
be very slow to list the directory.

Why did 311324ad1713 change nfs_readdir from

if (nfs_attribute_cache_expired(inode))
 nfs_revalidate_mapping(inode, file->f_mapping);
to

if (nfs_attribute_cache_expired(inode) || nfsi->cache_validity & NFS_INO_INVALID_DATA)
 nfs_revalidate_mapping(inode, file->f_mapping);

As the commit message says, the whole purpose was to use READDIRPLUS as a substitute for multiple GETATTR calls when the heuristic tells us that the user is performing an ‘ls -l’ style of workload.


.. and can we go back to the way it was before?

Not without slowing down ‘ls -l’ on large directories.


OK.. I understand why -- it is more correct since if we know the directory has changed, we might as well fetch the change. Otherwise, we might be creating
files and then wondering why they aren't listed.

It might be nicer to not invalidate the mapping we're currently using for readdir, though. Maybe there's a way to keep the mapping for the currently
opened directory and invalidate it once it's closed.


POSIX requires that you revalidate on opendir() and rewinddir(), and leaves behaviour w.r.t. file addition and removal after the call to
opendir()/rewinddir() undefined
(http://pubs.opengroup.org/onlinepubs/9699919799/functions/readdir.html).

It is only undefined whether the added or removed entry is returned.
Other entries still need to returned exactly once.

In this case we're restarting the read of the directory from scratch--I
don't understand how that's possible while avoiding skipped or
duplicated entries.

Surely the only safe thing to do is to continue reading using the last
cookie returned from the server.

Why? The client should be able to restart using any cookie at any time, and we rely on the cookies being unique to each entry. If you want more relaxed cookie semantics then be prepared to have to set up a stateful NFS
readdir protocol.

It looks to me as if I send in a non-zero position to nfs_readdir(), but the mapping
has been invalidated, then the client starts over at cookie 0 sending
READDIRs to the server and reading the directory entries into the page cache
in order to figure out which position maps to which entry/cookie.

Yes.

It seems possible to get duplicated or skipped entries.

There is a logical step missing here. How do you get from “we can end up re-filling the cache from scratch” to “there can be duplicate or skipped
entries”?

Because I thought we were searching for the position, not the cookie.  I
missed that we keep the last position/cookie around. Thanks, I need to read
more carefully.

It seems a seekdir would clear that cookie, then we might have
duplicate/missing entries.. but this is getting far from the original point. There's plenty of previous discussion on telldir/seekdir as you point out,
so I'll go off and read that.

Ben
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