Messages in this thread | | | Date | Mon, 5 Apr 2010 14:08:34 -0700 | From | Josh Triplett <> | Subject | Re: [PATCH] introduce sys_membarrier(): process-wide memory barrier (v10) |
| |
On Mon, Apr 05, 2010 at 03:10:57PM -0400, Mathieu Desnoyers wrote: > * Randy Dunlap (randy.dunlap@oracle.com) wrote: > > On Mon, 5 Apr 2010 13:57:37 -0400 Mathieu Desnoyers wrote: > > > +/* > > > + * sys_membarrier - issue memory barrier on current process running threads > > > + * @flags: One of these must be set: > > > + * MEMBARRIER_EXPEDITED > > > + * Adds some overhead, fast execution (few microseconds) > > > + * MEMBARRIER_DELAYED > > > + * Low overhead, but slow execution (few milliseconds) > > > + * > > > + * MEMBARRIER_QUERY > > > + * This optional flag can be set to query if the kernel supports > > > + * a set of flags. > > > + * > > > + * return values: Returns -EINVAL if the flags are incorrect. Testing for kernel > > > + * sys_membarrier support can be done by checking for -ENOSYS return value. > > > + * Return values >= 0 indicate success. For a given set of flags on a given > > > + * kernel, this system call will always return the same value. It is therefore > > > + * correct to check the return value only once at library load, passing the > > > > library load assumes caller is a library? does the kernel care about that? > > Nope, it doesn't. Will rephrase: > > ... It is therefore > * correct to check the return value only once during a process lifetime, > * passing the MEMBARRIER_QUERY flag in addition to only check if the flags are > * supported, without performing any synchronization.
Technically you can optimize even more than "process lifetime", since as you said the results hold "For a given set of flags on a given kernel". So you could check once and use the results as long as you remain on the same running system. (Craziness like live process migration and checkpoint/restart aside. :) )
- Josh Triplett
| |