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Support run-time configuration of the PIPE_MINDIRECT threshold.

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PIPE_MINDIRECT determines at what (blocking) write size one-copy
optimizations are applied in pipe(2) I/O.  That threshold hasn't
been tuned since the 1990s when this code was originally
committed, and allowing run-time reconfiguration will make it
easier to assess whether contemporary microarchitectures would
prefer a different threshold.

(On our local RPi4 baords, the 8k default would ideally be at least
32k, but it's not clear how generalizable that observation is.)

MFC after:	3 weeks
Reviewers:	jrtc27, arichardson
Differential Revision: https://reviews.freebsd.org/D29819
This commit is contained in:
Robert Watson 2021-01-02 16:42:28 +00:00
parent e68d76c054
commit af14713d49
1 changed files with 26 additions and 2 deletions
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28
sys/kern/sys_pipe.c
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@ -209,6 +209,7 @@ static int pipefragretry;
static int pipeallocfail;
static int piperesizefail;
static int piperesizeallowed = 1;
static long pipe_mindirect = PIPE_MINDIRECT;
SYSCTL_LONG(_kern_ipc, OID_AUTO, maxpipekva, CTLFLAG_RDTUN | CTLFLAG_NOFETCH,
&maxpipekva, 0, "Pipe KVA limit");
@ -263,6 +264,29 @@ pipeinit(void *dummy __unused)
KASSERT(pipedev_ino > 0, ("pipe dev inode not initialized"));
}
static int
sysctl_handle_pipe_mindirect(SYSCTL_HANDLER_ARGS)
{
int error = 0;
long tmp_pipe_mindirect = pipe_mindirect;
error = sysctl_handle_long(oidp, &tmp_pipe_mindirect, arg2, req);
if (error != 0 || req->newptr == NULL)
return (error);
/*
* Don't allow pipe_mindirect to be set so low that we violate
* atomicity requirements.
*/
if (tmp_pipe_mindirect <= PIPE_BUF)
return (EINVAL);
pipe_mindirect = tmp_pipe_mindirect;
return (0);
}
SYSCTL_OID(_kern_ipc, OID_AUTO, pipe_mindirect, CTLTYPE_LONG | CTLFLAG_RW,
&pipe_mindirect, 0, sysctl_handle_pipe_mindirect, "L",
"Minimum write size triggering VM optimization");
static int
pipe_zone_ctor(void *mem, int size, void *arg, int flags)
{
@ -1140,8 +1164,8 @@ pipe_write(struct file *fp, struct uio *uio, struct ucred *active_cred,
* away on us.
*/
if (uio->uio_segflg == UIO_USERSPACE &&
uio->uio_iov->iov_len >= PIPE_MINDIRECT &&
wpipe->pipe_buffer.size >= PIPE_MINDIRECT &&
uio->uio_iov->iov_len >= pipe_mindirect &&
wpipe->pipe_buffer.size >= pipe_mindirect &&
(fp->f_flag & FNONBLOCK) == 0) {
error = pipe_direct_write(wpipe, uio);
if (error != 0)