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From: "Nirmala S" <nmala@mail.com>
To: linux-kernel@vger.kernel.org
Date: Sun, 13 Apr 2003 07:54:37 -0500
Subject: Clustering of Request in block layer
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Hi,

As per my understanding the block layer clusters requests for all
block drivers.
Clustering -
Creating a linked list of buffer_heads using b_reqnext.

But, when I run my block driver and try to view the number of
clustered requests in my Request function, I do not find any
clustering done.

void own_request(request_queue_t *q)
{
    struct buffer_head *tmp;
    int count;
    while(1) {
        INIT_REQUEST;
        printk("<1>request %p: cmd %i sec %li (nr. %li)\n", CURRENT,
               CURRENT->cmd,
               CURRENT->sector,
               CURRENT->current_nr_sectors);
       count=0;
       for (tmp=bh; tmp; tmp=tmp->b_reqnext)
            count ++;
       printk("Count = %d\n", count);
        end_request(1); /* success */
    }
}

The above always shows 'Count = 1'. dd if=/dev/mydevice of=/dev/null.
Does this mean that no clustering is done ??

Just read a document a "http://www.usenix.org/publications/library/proceedings/usenix2000/freenix/full_papers/gopinath/gopinath_html/node14.html" 
which says "This clustering is performed only for the drivers compiled in the kernel and not for loadable modules."

Is this the reason ??

Regards,
Mala

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