linux-kernel - Re: [PATCH v1 4/5] zram: add swap full hint

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Date:	Wed, 24 Sep 2014 17:10:22 +0200
From:	Jerome Marchand <jmarchan@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Hugh Dickins <hughd@...gle.com>, Shaohua Li <shli@...nel.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Dan Streetman <ddstreet@...e.org>,
	Nitin Gupta <ngupta@...are.org>,
	Luigi Semenzato <semenzato@...gle.com>, juno.choi@....com
Subject: Re: [PATCH v1 4/5] zram: add swap full hint

On 09/23/2014 11:17 PM, Andrew Morton wrote:
> On Tue, 23 Sep 2014 13:56:02 +0900 Minchan Kim <minchan@...nel.org> wrote:
> 
>>>
>>>> +#define ZRAM_FULLNESS_PERCENT 80
>>>
>>> We've had problems in the past where 1% is just too large an increment
>>> for large systems.
>>
>> So, do you want fullness_bytes like dirty_bytes?
> 
> Firstly I'd like you to think about whether we're ever likely to have
> similar granularity problems with this tunable.  If not then forget
> about it.
> 
> If yes then we should do something.  I don't like the "bytes" thing
> much because it requires that the operator know the pool size
> beforehand, and any time that changes, the "bytes" needs hanging too. 
> Ratios are nice but percent is too coarse.  Maybe kernel should start
> using "ppm" for ratios, parts per million.  hrm.

An other possibility is to use decimal fractions. AFAIK, lustre fs uses
them already for its procfs entries.

> 
>>>> @@ -711,6 +732,7 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
>>>>  	down_write(&zram->init_lock);
>>>>  
>>>>  	zram->limit_pages = 0;
>>>> +	atomic_set(&zram->alloc_fail, 0);
>>>>  
>>>>  	if (!init_done(zram)) {
>>>>  		up_write(&zram->init_lock);
>>>> @@ -944,6 +966,34 @@ static int zram_slot_free_notify(struct block_device *bdev,
>>>>  	return 0;
>>>>  }
>>>>  
>>>> +static int zram_full(struct block_device *bdev, void *arg)
>>>
>>> This could return a bool.  That implies that zram_swap_hint should
>>> return bool too, but as we haven't been told what the zram_swap_hint
>>> return value does, I'm a bit stumped.
>>
>> Hmm, currently, SWAP_FREE doesn't use return and SWAP_FULL uses return
>> as bool so in the end, we can change it as bool but I want to remain it
>> as int for the future. At least, we might use it as propagating error
>> in future. Instead, I will use *arg to return the result instead of
>> return val. But I'm not strong so if you want to remove return val,
>> I will do it. For clarifictaion, please tell me again if you want.
> 
> I'm easy, as long as it makes sense, is understandable by people other
> than he-who-wrote-it and doesn't use argument names such as "arg".
> 
> 



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