On 23 Aug 2016, at 17:21, Benjamin Coddington 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);
.. and can we go back to the way it was before?
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.
But that won't happen unless a process deliberately starts over at
position
0.. I think a user creating a file, then listing the directory would
work
as expected.
Scott Mayhew already addressed this problem in:
07b5ce8ef2d8 ("NFS: Make nfs_readdir revalidate less often")
So, from 3.10 to 3.14 we had the behavior where we wouldn't clear the
mapping unless pos = 0 or the attribute cache expired.
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 readdir says:
If a file is removed from or added to the directory after the most
recent
call to opendir() or rewinddir(), whether a subsequent call to
readdir()
returns an entry for that file is unspecified.
However, RFC 1813 says:
The client should be careful to avoid holding directory entry
cookies
across operations that modify the directory contents, such as
REMOVE and
CREATE.
Ideally, we would keep using the same cookieverf until the server sends
NFS3ERR_BAD_COOKIE, and maybe just keep track of the last position/entry
cookie so we could pick up where we left off after invalidation.
Ben
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