high resolution timers, scheduling & sleep granularity

[Ric Wheeler <rwheeler@redhat.com>]

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?

Regards,

Ric