Bob Friesenhahn writes: > On Tue, 15 Apr 2008, Mark Maybee wrote: > > going to take 12sec to get this data onto the disk. This "impedance > > mis-match" is going to manifest as pauses: the application fills > > the pipe, then waits for the pipe to empty, then starts writing again. > > Note that this won't be smooth, since we need to complete an entire > > sync phase before allowing things to progress. So you can end up > > with IO gaps. This is probably what the original submitter is > > Yes. With an application which also needs to make best use of > available CPU, these I/O "gaps" cut into available CPU time (by > blocking the process) unless the application uses multithreading and > an intermediate write queue (more memory) to separate the CPU-centric > parts from the I/O-centric parts. While the single-threaded > application is waiting for data to be written, it is not able to read > and process more data. Since reads take time to complete, being > blocked on write stops new reads from being started so the data is > ready when it is needed. > > > There is one "down side" to this new model: if a write load is very > > "bursty", e.g., a large 5GB write followed by 30secs of idle, the > > new code may be less efficient than the old. In the old code, all > > This is also a common scenario. :-) > > Presumably the special "slow I/O" code would not kick in unless the > burst was large enough to fill quite a bit of the ARC. >
Bursts of 1/8th of physical memory or 5 seconds of storage throughput whichever is smallest. -r > Real time throttling is quite a challenge to do in software. > > Bob > ====================================== > Bob Friesenhahn > [EMAIL PROTECTED], http://www.simplesystems.org/users/bfriesen/ > GraphicsMagick Maintainer, http://www.GraphicsMagick.org/ > > _______________________________________________ > zfs-discuss mailing list > zfs-discuss@opensolaris.org > http://mail.opensolaris.org/mailman/listinfo/zfs-discuss _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
