On Wed, Nov 30, 2011 at 10:01:12PM +0100, Giovanni Trematerra wrote: > On Wed, Oct 5, 2011 at 6:56 PM, Konstantin Belousov <k...@freebsd.org> wrote: > > Author: kib > > Date: Wed Oct 5 16:56:06 2011 > > New Revision: 226042 > > URL: http://svn.freebsd.org/changeset/base/226042 > > > > Log: > > Supply unique (st_dev, st_ino) value pair for the fstat(2) done on the > > pipes. > > > > Reviewed by: jhb, Peter Jeremy <peterjeremy acm org> > > MFC after: 2 weeks > > > > Modified: > > head/sys/kern/sys_pipe.c > > head/sys/sys/pipe.h > > > > Hi Konstantin, > unfortunately your commit introduces a performance penalty of about 3% > documented below in a real workload. > I guess that fstat(2) on the pipes is seldom used so we could just be lazy > and alloc a new unr number inside pipe_stat instead of during pipe creation. > In that case if an application doesn't need fstat(2) on the pipes it > won't be charged > the cost of alloc_unr/free_unr for the fake inode number. > The patch I propose, furthermore, fix a panic in the case alloc_unr > failed to allocate > a new unr number and return -1. This is because ino_t is unsigned and the test > pipe_ino > 0 into pipeclose would be true, calling then free_unr when > it hasn't to. > The proposed patch was tested with a regression test code that you can find > here > > http://www.trematerra.net/patches/pipe-fstat.c > > Feel free to add it under tools/regression/pipe/ > > Here the proposed patch: > > http://www.trematerra.net/patches/lazy_inoalloc.diff > > Here the report of the benchmark: > > Configuration > 10.0-CURRENT updated to r22807. > kern.smp.disabled=1 in /boot/loader.conf > kernel config GENERIC without debugging options. > > The first result of any test was discarded and not reported here. > > here the result of three executions of > # make -s buildkernel > note that I managed to compile the same identical source files > for all the tests. > > r22807 with r226042 reverted (time in seconds) > 527, 527, 527 > > r22807 (time in seconds) > 544, 544, 544 > > r22807M with the proposed patch (time in seconds) > 527, 528, 528 > > ministat output: > > x r22807 with r226042 reverted > + r22807 > * r22807M with the proposed patch > +------------------------------------------------------------------------------+ > |+ * > x| > |* * > x| > ||__A_M| > A| > +------------------------------------------------------------------------------+ > N Min Max Median Avg Stddev > x 3 544 544 544 544 0 > + 3 527 527 527 527 0 > Difference at 95.0% confidence > -17 +/- 0 > -3.125% +/- 0% > (Student's t, pooled s = 0) > * 3 527 528 528 527.66667 0.57735027 > Difference at 95.0% confidence > -16.3333 +/- 0.925333 > -3.00245% +/- 0.170098% > (Student's t, pooled s = 0.408248) > > -- > Gianni Thank you for looking at this. I committed the test, and the fix for the call to free_unr(-1).
Regarding the lazy allocation of the inode number, I agree with the idea,
but have some reservations against the implementation. If several threads
call fstat(2) on the same pipe which inode is not yet initialized, then
I see a race in the patch. The easiest workaround is to cover the inode
allocation with the pipe lock.
Also, I find the return of ENOMEM from fstat(2) somewhat questionable. The
error code is not documented as allowed for the syscall. I prefer to
not fail the fstat(2) if lazy allocation failed, but return some fake
value for inode instead.
Updated patch is below.
diff --git a/sys/kern/sys_pipe.c b/sys/kern/sys_pipe.c
index 2b6eb66..19fc98f 100644
--- a/sys/kern/sys_pipe.c
+++ b/sys/kern/sys_pipe.c
@@ -569,12 +569,7 @@ pipe_create(pipe, backing)
/* If we're not backing this pipe, no need to do anything. */
error = 0;
}
- if (error == 0) {
- pipe->pipe_ino = alloc_unr(pipeino_unr);
- if (pipe->pipe_ino == -1)
- /* pipeclose will clear allocated kva */
- error = ENOMEM;
- }
+ pipe->pipe_ino = -1;
return (error);
}
@@ -1398,16 +1393,40 @@ pipe_stat(fp, ub, active_cred, td)
struct ucred *active_cred;
struct thread *td;
{
- struct pipe *pipe = fp->f_data;
+ struct pipe *pipe;
+ int new_unr;
#ifdef MAC
int error;
+#endif
+ pipe = fp->f_data;
PIPE_LOCK(pipe);
+#ifdef MAC
error = mac_pipe_check_stat(active_cred, pipe->pipe_pair);
- PIPE_UNLOCK(pipe);
- if (error)
+ if (error) {
+ PIPE_UNLOCK(pipe);
return (error);
+ }
#endif
+ /*
+ * Lazily allocate an inode number for the pipe. Most pipe
+ * users do not call stat(2) on the pipe, which means that
+ * postponing the inode allocation until it is must be returned to
+ * userland is useful. If alloc_unr failed, assign st_ino
+ * zero instead of returning an error.
+ * Special pipe_ino values:
+ * -1 - not yet initialized;
+ * 0 - alloc_unr failed, return 0 as st_ino forever.
+ */
+ if (pipe->pipe_ino == (ino_t)-1) {
+ new_unr = alloc_unr(pipeino_unr);
+ if (new_unr != -1)
+ pipe->pipe_ino = new_unr;
+ else
+ pipe->pipe_ino = 0;
+ }
+ PIPE_UNLOCK(pipe);
+
bzero(ub, sizeof(*ub));
ub->st_mode = S_IFIFO;
ub->st_blksize = PAGE_SIZE;
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