On Tue, Jun 16, 2015 at 1:05 AM, Tiankun Hu <tiankun...@foxmail.com> wrote:
> Hi Marcus, > Thanks your reply, I got it, it is really cool. > If just mapped to one address, in general_work function user need to > consider whether R/W pointer has out of buffer size, right? I wrote up a presentation explaining the stages of the scheduler in more depth: http://www.trondeau.com/blog/2013/9/15/explaining-the-gnu-radio-scheduler.html Tom > ------------------ Original ------------------ > *From: * "Marcus Müller";<marcus.muel...@ettus.com>; > *Date: * Mon, Jun 15, 2015 10:03 PM > *To: * "discuss-gnuradio"<discuss-gnuradio@gnu.org>; > *Subject: * Re: [Discuss-gnuradio] why block's buffer was mapped to two > address > > Hi Tiankun Hu, > > I hope I'm answering the right question, but: > gr-buffer maps the same pages into memory multiple times, forming a > pseudo-ring buffer, so that no matter where in your buffer you start, > you always get the full buffer length of "sequential data". This is very > cool for algorithms that expect consecutive samples, because there's no > need to find out where the next sample lies -- it's always at the > address right after the last one. > > To achieve that, GNU Radio gets pages that can be memory-mapped. For > example, under Linux, we get shared memory segments, and then mmap() > multiple times, right after the last mapping. > > > When do update reader/writer pointer, I found they was rounded to the > > real buffer size, so I want to know which code will use this second > > half address? > For example, your Buffer looks like the following: > W = 1kB written by the upstream block, unread (not consume()d) by the > downstream block > R = 1kB already consumed by the downstream block: > > W R R R W W > > so your read pointer is at the beginning of the fifth kilobyt: > > W R R R W W > ^ > read pointer > > Now , we'd like to let the reading block read the whole unread items at > once, i.e. all three W kilobytes. We simply do that by mapping a second > copy right after the first: > > W R R R W W|W R R R W W > ^ > read pointer > > After that call, the situation is: > > W W W W W W|W W W W W W > ^ > read pointer > > Of course, this isn't cool -- the read pointer should always be within > the buffer size, so we simply do a modulo buffer length : > > W W W W W W|W W W W W W > ^ > read pointer > > > Best regards, > Marcus > > > On 06/15/2015 03:48 PM, Tiankun Hu wrote: > > Hi, > > I found block's buffer was mapped to two continues address, is there > > any benefit for this? > > When do update reader/writer pointer, I found they was rounded to the > > real buffer size, so I want to know which code will use this second > > half address? > > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > >
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