Messages in this thread | | | Date | Mon, 28 Apr 2008 12:18:40 -0400 | Subject | Re: stat benchmark | From | "J. Bruce Fields" <> |
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On Mon, Apr 28, 2008 at 07:53:22AM -0400, Theodore Tso wrote: > On Sun, Apr 27, 2008 at 09:43:05PM -0700, Ulrich Drepper wrote: > > On Thu, Apr 24, 2008 at 1:59 PM, Soeren Sandmann <sandmann@daimi.au.dk> wrote: > > > So I am looking for ways to improve this. > > > > Aside from what has already been proposed there is also the > > readdirplus() route. Unfortunately the people behind this and related > > proposals vanished after the last discussions. I was hoping they > > come back with a revised proposal but perhaps not. Maybe it's time to > > pick up the ball myself. > > > > As a reminder, readdirplus() is an extended readdir() which also > > returns (a subset of) the stat information for the file at the same > > time. The subset part is needed to account for the different > > information contained in the inodes. For most applications the subset > > should be sufficient and therefore all that's needed is a single > > iteration over the directory. > > I'm not sure this would help in the cold cache case, which is what > Soeren originally complained about.[1] The problem is whaever > information the user might need won't be store in the directory, so > the filesystem would end having to stat the file anyway, incurring a > disk seek, which was what the user was complaining about. A > readdirplus() would save a whole bunch of system calls if the inode > was already cached, yes, but I'm not sure that's it would be worth the > effort given how small Linux's system call overhead would be. But in > the cold cache case, you end up seeking all over the disk, and the > only thing you can do is to try to keep the inodes close to each > other, and to have either readdir() or the caller of readdir() sort > all of the returned directory entries by inode number to avoid seeking > all over the disk.
The other reason for something like a readdirplus or a bulk stat is to provide an opportunity for parallelism.
As my favorite example: cold-cache "git diff" of a linux tree on my desktop (with an nfs-mounted /home) takes about 12 seconds. That's mainly just a sequential stat of about about 24000 files. Patching git to issue the stats in parallel, I could get that down to about 3.5 seconds. (Still not great. I don't know if it's disk seeks on the server or what that are the limiting factor.)
In the case of git, it's looking just for files that it tracks--it's not reading whole directories--so I don't know if readdirplus() specifically would help.
--b.
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