Re: [PATCHSET v5] Make background writeback great again for the first time

[xiakaixu]

ä 2016/4/28 4:59, Jens Axboe åé:
> On Wed, Apr 27 2016, Jens Axboe wrote:
>> On Wed, Apr 27 2016, Jens Axboe wrote:
>>> On 04/27/2016 12:01 PM, Jan Kara wrote:
>>>> Hi,
>>>>
>>>> On Tue 26-04-16 09:55:23, Jens Axboe wrote:
>>>>> Since the dawn of time, our background buffered writeback has sucked.
>>>>> When we do background buffered writeback, it should have little impact
>>>>> on foreground activity. That's the definition of background activity...
>>>>> But for as long as I can remember, heavy buffered writers have not
>>>>> behaved like that. For instance, if I do something like this:
>>>>>
>>>>> $ dd if=/dev/zero of=foo bs=1M count=10k
>>>>>
>>>>> on my laptop, and then try and start chrome, it basically won't start
>>>>> before the buffered writeback is done. Or, for server oriented
>>>>> workloads, where installation of a big RPM (or similar) adversely
>>>>> impacts database reads or sync writes. When that happens, I get people
>>>>> yelling at me.
>>>>>
>>>>> I have posted plenty of results previously, I'll keep it shorter
>>>>> this time. Here's a run on my laptop, using read-to-pipe-async for
>>>>> reading a 5g file, and rewriting it. You can find this test program
>>>>> in the fio git repo.
>>>>
>>>> I have tested your patchset on my test system. Generally I have observed
>>>> noticeable drop in average throughput for heavy background writes without
>>>> any other disk activity and also somewhat increased variance in the
>>>> runtimes. It is most visible on this simple testcases:
>>>>
>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000
>>>>
>>>> and
>>>>
>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
>>>>
>>>> The machine has 4GB of ram, /mnt is an ext3 filesystem that is freshly
>>>> created before each dd run on a dedicated disk.
>>>>
>>>> Without your patches I get pretty stable dd runtimes for both cases:
>>>>
>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000
>>>> Runtimes: 87.9611 87.3279 87.2554
>>>>
>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
>>>> Runtimes: 93.3502 93.2086 93.541
>>>>
>>>> With your patches the numbers look like:
>>>>
>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000
>>>> Runtimes: 108.183, 97.184, 99.9587
>>>>
>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
>>>> Runtimes: 104.9, 102.775, 102.892
>>>>
>>>> I have checked whether the variance is due to some interaction with CFQ
>>>> which is used for the disk. When I switched the disk to deadline, I still
>>>> get some variance although, the throughput is still ~10% lower:
>>>>
>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000
>>>> Runtimes: 100.417 100.643 100.866
>>>>
>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
>>>> Runtimes: 104.208 106.341 105.483
>>>>
>>>> The disk is rotational SATA drive with writeback cache, queue depth of the
>>>> disk reported in /sys/block/sdb/device/queue_depth is 1.
>>>>
>>>> So I think we still need some tweaking on the low end of the storage
>>>> spectrum so that we don't lose 10% of throughput for simple cases like
>>>> this.
>>>
>>> Thanks for testing, Jan! I haven't tried old QD=1 SATA. I wonder if
>>> you are seeing smaller requests, and that is why it both varies and
>>> you get lower throughput? I'll try and setup a test here similar to
>>> yours.
>>
>> Jan, care to try the below patch? I can't fully reproduce your issue on
>> a SCSI disk limited to QD=1, but I have a feeling this might help. It's
>> a bit of a hack, but the general idea is to allow one more request to
>> build up for QD=1 devices. That eliminates wait time between one request
>> finishing, and the next being submitted.
> 
> That accidentally added a potentially stall, this one is both cleaner
> and should have that fixed.
> 
> diff --git a/lib/wbt.c b/lib/wbt.c
> index 650da911f24f..322f5e04e994 100644
> --- a/lib/wbt.c
> +++ b/lib/wbt.c
> @@ -98,18 +98,23 @@ void __wbt_done(struct rq_wb *rwb)
>  	else
>  		limit = rwb->wb_normal;
Hi Jens,

This statement 'limit = rwb->wb_normal' is executed twice, maybe once is
enough. It is not a big deal anyway :)


Another question about this if branch:

   if (rwb->wc && !atomic_read(&rwb->bdi->wb.dirty_sleeping))
	limit = 0;

I can't follow the logic of this if branch. why set limit equal to 0
when the device supports write back caches and there are tasks being
limited in balance_dirty_pages(). Could you pelase give more info
about this ?  Thanks!
>  
> +	inflight = atomic_dec_return(&rwb->inflight);
> +
>  	/*
> -	 * Don't wake anyone up if we are above the normal limit. If
> -	 * throttling got disabled (limit == 0) with waiters, ensure
> -	 * that we wake them up.
> +	 * wbt got disabled with IO in flight. Wake up any potential
> +	 * waiters, we don't have to do more than that.
>  	 */
> -	inflight = atomic_dec_return(&rwb->inflight);
> -	if (limit && inflight >= limit) {
> -		if (!rwb->wb_max)
> -			wake_up_all(&rwb->wait);
> +	if (!rwb_enabled(rwb)) {
> +		wake_up_all(&rwb->wait);
>  		return;
>  	}

Maybe it is better that executing this if branch earlier. So we can wake up
potential waiters in time when wbt got disabled.
>  
> +	/*
> +	 * Don't wake anyone up if we are above the normal limit.
> +	 */
> +	if (inflight && inflight >= limit)
> +		return;
> +
>  	if (waitqueue_active(&rwb->wait)) {
>  		int diff = limit - inflight;
>  
> @@ -150,14 +155,26 @@ static void calc_wb_limits(struct rq_wb *rwb)
>  		return;
>  	}
>  
> -	depth = min_t(unsigned int, RWB_MAX_DEPTH, rwb->queue_depth);
> -
>  	/*
> -	 * Reduce max depth by 50%, and re-calculate normal/bg based on that
> +	 * For QD=1 devices, this is a special case. It's important for those
> +	 * to have one request ready when one completes, so force a depth of
> +	 * 2 for those devices. On the backend, it'll be a depth of 1 anyway,
> +	 * since the device can't have more than that in flight.
>  	 */
> -	rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
> -	rwb->wb_normal = (rwb->wb_max + 1) / 2;
> -	rwb->wb_background = (rwb->wb_max + 3) / 4;
> +	if (rwb->queue_depth == 1) {
> +		rwb->wb_max = rwb->wb_normal = 2;
> +		rwb->wb_background = 1;
> +	} else {
> +		depth = min_t(unsigned int, RWB_MAX_DEPTH, rwb->queue_depth);
> +
> +		/*
> +		 * Reduce max depth by 50%, and re-calculate normal/bg based on
> +		 * that.
> +		 */
> +		rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
> +		rwb->wb_normal = (rwb->wb_max + 1) / 2;
> +		rwb->wb_background = (rwb->wb_max + 3) / 4;
> +	}
>  }
>  
>  static bool inline stat_sample_valid(struct blk_rq_stat *stat)
> 


-- 
Regards
Kaixu Xia