On Wed, Oct 11, 2017 at 08:48:40PM +0200, Peter Zijlstra wrote: > On Wed, Oct 11, 2017 at 02:01:51PM -0400, Waiman Long wrote: > > # of Patches Reader Writer > > Applied Locking Rate Locking Rate > > ------------ ------------ ------------ > > 0 5,155/ 5,155/ 5,155 5,154/248,852/346,281 > > 7 5,696/ 5,697/ 5,698 113,500/215,826/320,872 > > 8 4,827/ 5,047/ 5,215 4,826/176,797/284,069 > > 9 211,276/ 509,712/1,134,007 4,894/221,839/246,818 > > 11 884,513/1,043,989/1,252,533 9,604/ 11,105/ 25,225 > > > > It can be seen that rwsem changes from writer-preferring to > > reader-preferring. > > A bit radically so, you almost starve the writers there. Which is a bit of a problem for us, because we often use the write locks as an IO barrier for operations like truncate, fallocate, etc. i.e. we want it to immediately block readers. That's going to be a bit of a problem if, for example, we have so many AIO-based direct IO writers on a file we can't get fallocate to run in a timely fashion to preallocate the space the writers are soon going to write into. Not to mention the AIO-DIO append case where we have multiple concurrent writers at EOF, and so every so often one of the many IOs needs to take the write lock extending EOF safely. Blocking that for 10ms waiting for a hand-off is going to make all the people who care about deterministic IO latency go nuts.... So from my perspective on the IO side, I'd much prefer a write bias. Indeed, if we go back to the Irix XFS code, all these locks we defined as "MR_BARRIER" locks, which meant the XFS rwsems were specifically intended to have writer bias. I think we can live with a fair r/w bias, but swinging from a 50:1 write bias to a 100:1 read bias is going change behaviour dramatically, and in many cases it won't be an improvement... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-alpha" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
