Re: [Gluster-devel] stack_wind questions

[Raghavendra G]

Hi Corin,

STACK_WIND and STACK_UNWIND are analogous to C procedure call and return from it. But, the major difference is that in C when a return is done from a procedure, the control is returned to the calling procedure, but when a STACK_UNWIND is done, the control is returned to the procedure provided as an argument to STACK_WIND macro (the "cbk" argument). The STACK_WIND macro stores the "context" (call back procedure etc) necessary to do this. A series of STACK_WIND macros results in a stack of these contexts built on heap and the corresponding STACK_UNWINDs results in unwinding of this stack (Note that the stack built due to calls to STACK_WIND is different from the C function stack, which is cleared as soon as the C function returns. But the "context"/stack built by STACK_WIND is preserved across C functions). 

In other words, STACK_WIND/STACK_UNWIND implements continuations (provided by lisp and other languages) for glusterfs in C.

These two macros are among the basic building blocks of glusterfs' asynchronous model of operation.

STACK_WIND/STACK_UNWIND pair of macros helps to handle the operations across the network (say between client and server) asynchronously. The request/reply for/to an operation is written to network, but the glusterfs is not blocked until the response is returned. Instead it "pauses the current operation" and continues to act upon requests for other operations. When the response is got, the corresponding "paused operation" is resumed using the stack built by series of STACK_WINDs till the request/reply was written to network.

regards,

On Thu, Oct 30, 2008 at 2:58 AM, Corin Langosch <address@hidden> wrote:

Hi,

I just looked some time at the code but I think I just didn't get the
usage of STACK_WIND / STACK_UNWIND right.

Looking at the macros and the translators (for example), STACK_WIND
simply seems to setup some datastructures and then call the suplied
function. After that function is done, the MACRO is done and code
executions continues normal.

So if I put a STACK_WIND into a loop (like in unify) the function passed
in the MACRO is simply called. So the calls don't happen in parallel but
normal seriazliezd, as they would have happened without using
STACK_WIND. So what is STACK_WIND all about - for me it currently seems
to be for passing (some common) data between function calls. It doesn't
execute any functions in parallel in order to reduce latencies caused by
the backends?

The function call in the STACK_UNWIND macro puzzles me even more. What
is this for? As the STACK_UNWIND function is called from within the
function called by STACK_WIND, I'd suspect some kind of loop?

What's about with the while(0) inside the macros. They don't do anything? ;)

Thanks for any clarifications :)
Corin


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-- 
Raghavendra G