[PATCH 5/6] f2fs: add a kernel thread to issue discard commands asynchronously
Jaegeuk Kim
jaegeuk at kernel.org
Wed Feb 8 14:05:40 PST 2017
On 02/08, Christoph Hellwig wrote:
> On Mon, Feb 06, 2017 at 07:44:03PM -0800, Jaegeuk Kim wrote:
> > Sorry for the late response due to the travel.
> >
> > When doing fstrim with a fresh f2fs image fomatted on Intel NVMe SSD whose
> > model name is SSDPE2MW012T4, I've got the following trace.
>
> <snip>
>
> > So, I investigated why block_rq_complete() happened in more detail.
> >
> > The root-caused call path looks like:
> > - submit_bio
> > - generic_make_request
> > - q->make_request_fn
> > - blk_mq_make_request
> > - blk_mq_map_request
> > - blk_mq_alloc_request
> > - blk_mq_get_tag
> > - __blk_mq_get_tag
> > - bt_get
> > - blk_mq_run_hw_queue
> > - finish_wait
> > --> this waits for pending 8 discard bios!
>
> You're blocking on tag allocation. How many tags per queue does
> your device have?, e.g. do a
>
> cat /sys/block/nvme0n1/mq/0/nr_tags
It shows 1023.
> > It seems the problem comes from the storage processing discard commands too
> > slowly comparing to normal read/write IOs.
> >
> > Any thoughts?
>
> Deallocate is always going to be an exception path compared to normal
> read/write… but just how much slower is going to be device
> dependent.
>
> One option would be to reuse the number of discards, for that can you
> try the series here to support vectored discards:
>
> http://git.infradead.org/users/hch/block.git/shortlog/refs/heads/vectored-discard-for-axboe
I tried this, but couldn't see any difference.
Thanks,
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