linux-kernel - high resolution timers, scheduling & sleep granularity

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Message-ID: <4892FC11.4020105@redhat.com>
Date:	Fri, 01 Aug 2008 08:05:37 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	Josef Bacik <jbacik@...hat.com>, linux-fsdevel@...r.kernel.org,
	Chris Mason <chris.mason@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: high resolution timers, scheduling & sleep granularity

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

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