Hi, I am calling the merge function of the block device driver below me(since mine is only pass through). Does this not work? When i tried seeing what read requests were coming then i saw that when i issue dd with count=1 it retrieves 4 pages, so i tried with 'direct' flag. But even with direct io my read performance is way lower than my write performance. Regards, Neha On Wed, Apr 10, 2013 at 11:15 PM, Rajat Sharma <fs.rajat@gmail.com> wrote:
Hi,
On Thu, Apr 11, 2013 at 2:23 AM, neha naik <nehanaik27@gmail.com> wrote:
Hi All, Nobody has replied to my query here. So i am just wondering if there is a forum for block device driver where i can post my query. Please tell me if there is any such forum.
Thanks, Neha
---------- Forwarded message ---------- From: neha naik <nehanaik27@gmail.com> Date: Tue, Apr 9, 2013 at 10:18 AM Subject: Passthrough device driver performance is low on reads compared to writes To: kernelnewbies@kernelnewbies.org
Hi All, I have written a passthrough block device driver using 'make_request' call. This block device driver simply passes any request that comes to it down to lvm.
However, the read performance for my passthrough driver is around 65MB/s (measured through dd) and write performance is around 140MB/s for dd block size 4096. The write performance matches with lvm's write performance more or less but, the read performance on lvm is around 365MB/s.
I am posting snippets of code which i think are relevant here:
static int passthrough_make_request( struct request_queue * queue, struct bio * bio) {
passthrough_device_t * passdev = queue->queuedata; bio->bi_bdev = passdev->bdev_backing; generic_make_request(bio); return 0; }
For initializing the queue i am using following:
blk_queue_make_request(passdev->queue, passthrough_make_request); passdev->queue->queuedata = sbd; passdev->queue->unplug_fn = NULL; bdev_backing = passdev->bdev_backing; blk_queue_stack_limits(passdev->queue, bdev_get_queue(bdev_backing)); if ((bdev_get_queue(bdev_backing))->merge_bvec_fn) { blk_queue_merge_bvec(sbd->queue, sbd_merge_bvec_fn); }
What is the implementation for sbd_merge_bvec_fn? Please debug through it to check requests are merging or not? May be that is the cause of lower performance?
Now, I browsed through dm code in kernel to see if there is some flag or something which i am not using which is causing this huge performance penalty. But, I have not found anything.
If you have any ideas about what i am possibly doing wrong then please tell me.
Thanks in advance.
Regards, Neha
-Rajat
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