Re: Errors in the VM - detailed

[Jens Axboe <axboe@suse.de>]

On Thu, Jan 31 2002, Roger Larsson wrote:
> Wait a minute - it might be readahead that gets killed.
> If I remember correctly READA requests are dropped when failing to allocate 
> space for it - yes I did...

s/allocate/retrieve

No allocation takes place.

> /usr/src/develop/linux/drivers/block/ll_rw_block.c:746 (earlier kernel)
> 
> 	/*
> 	 * Grab a free request from the freelist - if that is empty, check
> 	 * if we are doing read ahead and abort instead of blocking for
> 	 * a free slot.
> 	 */
> get_rq:
> 	if (freereq) {
> 		req = freereq;
> 		freereq = NULL;
> 	} else if ((req = get_request(q, rw)) == NULL) {
> 		spin_unlock_irq(&io_request_lock);
> 		if (rw_ahead)
> 			goto end_io;
> 
> 		freereq = __get_request_wait(q, rw);
> 		goto again;
> 	}
> 
> Suppose we fail with get_request, the request is a rw_ahead,
> it quits... => no read ahead.
> 
> Try to add a prink there...
> 		if (rw_ahead) {
> 			printk("Skipping readahead...\n");
> 			goto end_io;
> 		}

That will trigger _all the time_ even on a moderately busy machine.
Checking if tossing away read-ahead is the issue is probably better
tested with just increasing the request slots. Roy, please try and change
the queue_nr_requests assignment in ll_rw_blk:blk_dev_init() to
something like 2048.

-- 
Jens Axboe