Ah-- re #1 -- I was investigating earlier why not as much was combined
as I thought should be when idle. This is surely a factor. Thanks
for the catch-- KEY_OFFSET is correct. I will fix and retest.
(Under heavy load, the correct thing still happens, but not under
light or intermediate load0.
About #2-- I wanted to attain a bounded amount of "combining" of
operations. If we have 5 4k extents in a row to dispatch, it seems
really wasteful to issue them as 5 IOs 60ms apart, which the existing
code would be willing to do-- I'd rather do a 20k write IO (basically
the same cost as a 4k write IO) and then sleep 300ms. It is dependent
on the elevator/IO scheduler merging the requests. At the same time,
I'd rather not combine a really large request.
It would be really neat to blk_plug the backing device during the
write issuance, but that requires further work.
Thanks
Mike
On Tue, Sep 26, 2017 at 11:51 PM, <tang.junhui@xxxxxxxxxx> wrote:
> From: Tang Junhui <tang.junhui@xxxxxxxxxx>
>
> Hello Lyle:
>
> Two questions:
> 1) In keys_contiguous(), you judge I/O contiguous in cache device, but not
> in backing device. I think you should judge it by backing device (remove
> PTR_CACHE() and use KEY_OFFSET() instead of PTR_OFFSET()?).
>
> 2) I did not see you combine samll contiguous I/Os to big I/O, so I think
> it is useful when writeback rate was low by avoiding single I/O write, but
> have no sense in high writeback rate, since previously it is also write
> I/Os asynchronously.
>
> -----------
> Tang Junhui
>
>> Previously, there was some logic that attempted to immediately issue
>> writeback of backing-contiguous blocks when the writeback rate was
>> fast.
>>
>> The previous logic did not have any limits on the aggregate size it
>> would issue, nor the number of keys it would combine at once. It
>> would also discard the chance to do a contiguous write when the
>> writeback rate was low-- e.g. at "background" writeback of target
>> rate = 8, it would not combine two adjacent 4k writes and would
>> instead seek the disk twice.
>>
>> This patch imposes limits and explicitly understands the size of
>> contiguous I/O during issue. It also will combine contiguous I/O
>> in all circumstances, not just when writeback is requested to be
>> relatively fast.
>>
>> It is a win on its own, but also lays the groundwork for skip writes to
>> short keys to make the I/O more sequential/contiguous.
>>
>> Signed-off-by: Michael Lyle <mlyle@xxxxxxxx>
>> ---
>> drivers/md/bcache/bcache.h | 6 --
>> drivers/md/bcache/writeback.c | 131 ++++++++++++++++++++++++++++++------------
>> 2 files changed, 93 insertions(+), 44 deletions(-)
>>
>> diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
>> index eb83be693d60..da803a3b1981 100644
>> --- a/drivers/md/bcache/bcache.h
>> +++ b/drivers/md/bcache/bcache.h
>> @@ -321,12 +321,6 @@ struct cached_dev {
>> struct bch_ratelimit writeback_rate;
>> struct delayed_work writeback_rate_update;
>>
>> - /*
>> - * Internal to the writeback code, so read_dirty() can keep track of
>> - * where it's at.
>> - */
>> - sector_t last_read;
>> -
>> /* Limit number of writeback bios in flight */
>> struct semaphore in_flight;
>> struct task_struct *writeback_thread;
>> diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
>> index 0b7c89813635..cf29c44605b9 100644
>> --- a/drivers/md/bcache/writeback.c
>> +++ b/drivers/md/bcache/writeback.c
>> @@ -229,10 +229,26 @@ static void read_dirty_submit(struct closure *cl)
>> continue_at(cl, write_dirty, io->dc->writeback_write_wq);
>> }
>>
>> +static inline bool keys_contiguous(struct cached_dev *dc,
>> + struct keybuf_key *first, struct keybuf_key *second)
>> +{
>> + if (PTR_CACHE(dc->disk.c, &second->key, 0)->bdev !=
>> + PTR_CACHE(dc->disk.c, &first->key, 0)->bdev)
>> + return false;
>> +
>> + if (PTR_OFFSET(&second->key, 0) !=
>> + (PTR_OFFSET(&first->key, 0) + KEY_SIZE(&first->key)))
>> + return false;
>> +
>> + return true;
>> +}
>> +
>> static void read_dirty(struct cached_dev *dc)
>> {
>> unsigned delay = 0;
>> - struct keybuf_key *w;
>> + struct keybuf_key *next, *keys[5], *w;
>> + size_t size;
>> + int nk, i;
>> struct dirty_io *io;
>> struct closure cl;
>>
>> @@ -243,45 +259,84 @@ static void read_dirty(struct cached_dev *dc)
>> * mempools.
>> */
>>
>> - while (!kthread_should_stop()) {
>> -
>> - w = bch_keybuf_next(&dc->writeback_keys);
>> - if (!w)
>> - break;
>> -
>> - BUG_ON(ptr_stale(dc->disk.c, &w->key, 0));
>> -
>> - if (KEY_START(&w->key) != dc->last_read ||
>> - jiffies_to_msecs(delay) > 50)
>> - while (!kthread_should_stop() && delay)
>> - delay = schedule_timeout_interruptible(delay);
>> -
>> - dc->last_read = KEY_OFFSET(&w->key);
>> -
>> - io = kzalloc(sizeof(struct dirty_io) + sizeof(struct bio_vec)
>> - * DIV_ROUND_UP(KEY_SIZE(&w->key), PAGE_SECTORS),
>> - GFP_KERNEL);
>> - if (!io)
>> - goto err;
>> -
>> - w->private = io;
>> - io->dc = dc;
>> -
>> - dirty_init(w);
>> - bio_set_op_attrs(&io->bio, REQ_OP_READ, 0);
>> - io->bio.bi_iter.bi_sector = PTR_OFFSET(&w->key, 0);
>> - bio_set_dev(&io->bio, PTR_CACHE(dc->disk.c, &w->key, 0)->bdev);
>> - io->bio.bi_end_io = read_dirty_endio;
>> -
>> - if (bio_alloc_pages(&io->bio, GFP_KERNEL))
>> - goto err_free;
>> -
>> - trace_bcache_writeback(&w->key);
>> + next = bch_keybuf_next(&dc->writeback_keys);
>> +
>> + while (!kthread_should_stop() && next) {
>> + size = 0;
>> + nk = 0;
>> +
>> + do {
>> + BUG_ON(ptr_stale(dc->disk.c, &next->key, 0));
>> +
>> + /* Don't combine too many operations, even if they
>> + * are all small.
>> + */
>> + if (nk >= 5)
>> + break;
>> +
>> + /* If the current operation is very large, don't
>> + * further combine operations.
>> + */
>> + if (size > 5000)
>> + break;
>> +
>> + /* Operations are only eligible to be combined
>> + * if they are contiguous.
>> + *
>> + * TODO: add a heuristic willing to fire a
>> + * certain amount of non-contiguous IO per pass,
>> + * so that we can benefit from backing device
>> + * command queueing.
>> + */
>> + if (nk != 0 && !keys_contiguous(dc, keys[nk-1], next))
>> + break;
>> +
>> + size += KEY_SIZE(&next->key);
>> + keys[nk++] = next;
>> + } while ((next = bch_keybuf_next(&dc->writeback_keys)));
>> +
>> + /* Now we have gathered a set of 1..5 keys to write back. */
>> +
>> + for (i = 0; i < nk; i++) {
>> + w = keys[i];
>> +
>> + io = kzalloc(sizeof(struct dirty_io) +
>> + sizeof(struct bio_vec) *
>> + DIV_ROUND_UP(KEY_SIZE(&w->key), PAGE_SECTORS),
>> + GFP_KERNEL);
>> + if (!io)
>> + goto err;
>> +
>> + w->private = io;
>> + io->dc = dc;
>> +
>> + dirty_init(w);
>> + bio_set_op_attrs(&io->bio, REQ_OP_READ, 0);
>> + io->bio.bi_iter.bi_sector = PTR_OFFSET(&w->key, 0);
>> + bio_set_dev(&io->bio,
>> + PTR_CACHE(dc->disk.c, &w->key, 0)->bdev);
>> + io->bio.bi_end_io = read_dirty_endio;
>> +
>> + if (bio_alloc_pages(&io->bio, GFP_KERNEL))
>> + goto err_free;
>> +
>> + trace_bcache_writeback(&w->key);
>> +
>> + down(&dc->in_flight);
>> +
>> + /* We've acquired a semaphore for the maximum
>> + * simultaneous number of writebacks; from here
>> + * everything happens asynchronously.
>> + */
>> + closure_call(&io->cl, read_dirty_submit, NULL, &cl);
>> + }
>>
>> - down(&dc->in_flight);
>> - closure_call(&io->cl, read_dirty_submit, NULL, &cl);
>> + delay = writeback_delay(dc, size);
>>
>> - delay = writeback_delay(dc, KEY_SIZE(&w->key));
>> + while (!kthread_should_stop() && delay) {
>> + schedule_timeout_interruptible(delay);
>> + delay = writeback_delay(dc, 0);
>> + }
>> }
>>
>> if (0) {
>> --
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