From: Paul H. Hargrove (PHHargrove_at_lbl_dot_gov)
Date: Thu Jun 18 2009 - 23:59:50 PDT
In "test_and_set_bit(0, &var)" the "0" is NOT a value to set. It tells
which bit in var should be set to 1. There is also a
test_and_clear_bit() which assigns "0" to the specified bit. Both of
these calls return the previous value of the bit they write to. So, each
call to test_and_set_bit(0, &var) sets bit number 0 of var and returns
its previous value. As used in the code you are quoting from, this means
that the first task to call "!test_and_set_bit(0,&var)" for a given
"var" returns 1 (because the bit was 0 before) while every call after
the first returns 0 (because the bit is now 1, and remains 1). One could
achieve the same thing with "!atomic_swap(&var,1)", where this time the
"1" IS the value to assign to var.
In the older BLCR version you mention, we were only able to deal with
single-process checkpoints. In more current versions we are able to deal
multi-process checkpoints and the "proc_req" is, as you have guessed,
the structure that holds the per-process information in a checkpoint or
restart request. So, each "req" includes portions that are independent
or common to all processes, and a list of "proc_req" structures that
contain the per-process information (which was contained in "req" when
we only dealt with single processes).
-Paul
����� wrote:
> Hello, Professor:
>
> How can we ensure information can be dumped only once or per process ?
>
> For example :
>
> the file header about scope & version , we need to write just once.
>
> Resources that are shared among the threads (such as memory mappings
> and file descriptors) are written out only once, by the first thread.
> we write it to the file per process.
>
> I see the code in dump_self.c , not the way I supposed to be.
>
> down(&req->serial_mutex);
>
> if (!test_and_set_bit(0, &req->done_header)) {
>
> result = cr_save_file_header(req, dest_filp);
> ...
> }
> up(&req->serial_mutex);
>
> there are many bool variable in the structure "req" and "proc_req" ,
> like done_header, but every time a thread come to execute the code
> section above, they can enter and do "cr_save_file_header", for
> "done_header" is set to "0" again...
>
> By the way, In the early version of BLCR, there is just one structure
> "req"(instanse of that struct, for convenience). In the later version,
> "proc_req" is introduced. Once I supposed structure "proc_req" is used
> to handle something per process. But now it seems not right. Please
> tell me the main goal of the introducion of "proc_req"?
>
> Thank you very much
>
>
>
>
>
>
>
> ===============================================
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>
--
Paul H. Hargrove PHHargrove_at_lbl_dot_gov
Future Technologies Group Tel: +1-510-495-2352
HPC Research Department Fax: +1-510-486-6900
Lawrence Berkeley National Laboratory