[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 <corinl at gmx.de> 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
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