On Fri, Aug 01, 2008 at 09:57:33AM -0400, Ric Wheeler wrote: > Josef Bacik wrote: >> On Fri, Aug 01, 2008 at 08:05:37AM -0400, Ric Wheeler wrote: >> >>> Hi Thomas & Ingo, >>> >>> Josef has been working on some patches to try and get ext3/4 to >>> dynamically detect the latency of a storage device and use that base >>> latency to tune the amount of time we sleep waiting for others to join in >>> a transaction. The logic in question lives in jbd/transaction.c >>> (transaction_stop). >>> >>> The code was originally developed to try and allow multiple threads to >>> join in a big, slow transaction. For example, transacations that write to >>> a slow ATA or S-ATA drive take in the neighborhood of 10 to 20 ms. >>> >>> Faster devices, for example a disk array, can complete the transaction >>> in 1.3 ms. Even higher speed SSD devices boast of a latency of 0.1ms, not >>> to mention RAM disks ;-) >>> >>> The current logic makes us wait way too long, especially with a 250HZ >>> kernel since we sleep many times longer than it takes to complete the IO >>> ;-) >>> >>> Do either of you have any thoughts on how to get a better, fine grained >>> sleep capability that we could use that would allow us to sleep in >>> sub-jiffie chunks? >>> >>> >> >> Hello, >> >> This is the most recent iteration of my patch using hrtimers. It works really >> well for ramdisks, so anything with low latency writes is going to be really >> fast, but I'm still trying to come up with a smart way to sleep long enough to >> not hurt SATA performance. As it stands now I'm getting a 5% decrease in speed >> on SATA. So I think I've got the sleep as little as possible part down right, >> just can't quite get it to sleep long enough if the disk is slow. Thanks, >> >> Josef >> > > I think that this (or similar) kind of precision_sleep() should be > generically useful. > > One question on the code, would it be better to measure the average > transaction time in the same units as your precision sleep uses - aren't > jiffies are still too coarse? > Oh well crap I guess thats why I'm not sleeping long enough, i'm only sleeping jiffies number of nanoseconds, and jiffies is much higher than nanoseconds... /me writes jiffies * HZ = seconds backwards on his forehead Josef -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
