On Tue, Jan 09, 2018 at 09:10:59AM -0500, Jeff Layton wrote: > From: Jeff Layton <jlayton@xxxxxxxxxx> > > Since i_version is mostly treated as an opaque value, we can exploit that > fact to avoid incrementing it when no one is watching. With that change, > we can avoid incrementing the counter on writes, unless someone has > queried for it since it was last incremented. If the a/c/mtime don't > change, and the i_version hasn't changed, then there's no need to dirty > the inode metadata on a write. > > Convert the i_version counter to an atomic64_t, and use the lowest order > bit to hold a flag that will tell whether anyone has queried the value > since it was last incremented. > > When we go to maybe increment it, we fetch the value and check the flag > bit. If it's clear then we don't need to do anything if the update > isn't being forced. > > If we do need to update, then we increment the counter by 2, and clear > the flag bit, and then use a CAS op to swap it into place. If that > works, we return true. If it doesn't then do it again with the value > that we fetch from the CAS operation. > > On the query side, if the flag is already set, then we just shift the > value down by 1 bit and return it. Otherwise, we set the flag in our > on-stack value and again use cmpxchg to swap it into place if it hasn't > changed. If it has, then we use the value from the cmpxchg as the new > "old" value and try again. > > This method allows us to avoid incrementing the counter on writes (and > dirtying the metadata) under typical workloads. We only need to increment > if it has been queried since it was last changed. > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx> > Reviewed-by: Jan Kara <jack@xxxxxxx> Documentation helps a lot in understanding all this. Thanks for adding it into the patch! Acked-by: Dave Chinner <dchinner@xxxxxxxxxx> -- Dave Chinner david@xxxxxxxxxxxxx
