On Fri, Jul 15, 2011 at 11:31 PM, Amirali Shambayati < amirali.shambayati@gmail.com> wrote:
Hi all,
As much as I know, type of an IO request can be realized by "DATA_DIR" macro,but I wanted to know if it is possible to distinguish a read or write is synchronous or asychronous.
I'm also using "blktrace" to extract requests dispatching to disk. blktrace, determines synchrnous writes as "ws" and asynchronous writes as "w".
at the electronics level, it is much easier to design things to run synchronously (another similar term is "isochronous" operation, eg, http://h10032.www1.hp.com/ctg/Manual/bph07333.pdf), and from many hardware timing diagram, u can see these making transition based on different signals received - behaving in a deterministic way to complete a state transition machine. ie, a read is a read, and not "read later". it is the software that does the asynchronous operation - "read" becomes "read later" - thus opening a window of duration where data may not be written as requested - that is the tradeoff or sacrifices for async operation. historically, async I/O comes later (http://lwn.net/Articles/94566/) as it is quite difficult to implement (but it do have the feature of non-blocking, http://en.wikipedia.org/wiki/Asynchronous_I/O). there is the issue of reordering the data read if multiple read happens within the latency window. PCI do have a hardware feature called "write posting" - whereby multiple read/write can be submitted, and the actual operation only happen in a burst mode - normally reading multiple blocks at the same time. i may be wrong somewhere above, but at the low level (device driver) it may not possible to distinguish between sync or async operation - unless the hardware has the ability to report on the mode it is currently set - and from software perspective we can deduce it being synchronous or asynchronous. (eg, if write posting is enabled in hardware, then async is enabled - make sense?) criticism welcomed :-).
Regards,
-- Amirali Shambayati Bachelor Student Computer Engineering Department Sharif University of Technology Tehran, Iran
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-- Regards, Peter Teoh