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Subject: Re: [PATCH] zswap: Add shrink_enabled that can disable swap shrink to
 increase store performance
To:     Dan Streetman <ddstreet@ieee.org>
Cc:     Seth Jennings <sjenning@redhat.com>, Linux-MM <linux-mm@kvack.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
References: <1571990538-6133-1-git-send-email-teawaterz@linux.alibaba.com>
 <CALZtONCQ1YqpAXfZS6jemHuKpBXhLz440EcxSoWZbxrH0kyLHg@mail.gmail.com>
From:   Hui Zhu <teawaterz@linux.alibaba.com>
Message-ID: <42753fc6-e352-adcb-52c2-6b68472318f5@linux.alibaba.com>
Date:   Mon, 11 Nov 2019 09:57:53 +0800
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On 2019/11/9 12:04 上午, Dan Streetman wrote:
> On Fri, Oct 25, 2019 at 4:02 AM Hui Zhu <teawaterz@linux.alibaba.com> wrote:
>>
>> zswap will try to shrink pool when zswap is full.
>> This commit add shrink_enabled that can disable swap shrink to increase
>> store performance.  User can disable swap shrink if care about the store
>> performance.
> 
> I don't understand - if zswap is full it can't store any more pages
> without shrinking the current pool.  This commit will just force all
> pages to swap when zswap is full.  This has nothing to do with 'store
> performance'.
> 
> I think it would be much better to remove any user option for this and
> implement some hysteresis; store pages normally until the zpool is
> full, then reject all pages going to that pool until there is some %
> free, at which point allow pages to be stored into the pool again.
> That will prevent (or at least reduce) the constant performance hit
> when a zpool fills up, and just fallback to normal swapping to disk
> until the zpool has some amount of free space again.
> 

This idea is really cool!
Do you mind I make a patch for it?

Thanks,
Hui

>>
>> For example in a VM with 1 CPU 1G memory 4G swap:
>> echo lz4 > /sys/module/zswap/parameters/compressor
>> echo z3fold > /sys/module/zswap/parameters/zpool
>> echo 0 > /sys/module/zswap/parameters/same_filled_pages_enabled
>> echo 1 > /sys/module/zswap/parameters/enabled
>> usemem -a -n 1 $((4000 * 1024 * 1024))
>> 4718592000 bytes / 114937822 usecs = 40091 KB/s
>> 101700 usecs to free memory
>> echo 0 > /sys/module/zswap/parameters/shrink_enabled
>> usemem -a -n 1 $((4000 * 1024 * 1024))
>> 4718592000 bytes / 8837320 usecs = 521425 KB/s
>> 129577 usecs to free memory
>>
>> The store speed increased when zswap shrink disabled.
>>
>> Signed-off-by: Hui Zhu <teawaterz@linux.alibaba.com>
>> ---
>>   mm/zswap.c | 7 +++++++
>>   1 file changed, 7 insertions(+)
>>
>> diff --git a/mm/zswap.c b/mm/zswap.c
>> index 46a3223..731e3d1e 100644
>> --- a/mm/zswap.c
>> +++ b/mm/zswap.c
>> @@ -114,6 +114,10 @@ static bool zswap_same_filled_pages_enabled = true;
>>   module_param_named(same_filled_pages_enabled, zswap_same_filled_pages_enabled,
>>                     bool, 0644);
>>
>> +/* Enable/disable zswap shrink (enabled by default) */
>> +static bool zswap_shrink_enabled = true;
>> +module_param_named(shrink_enabled, zswap_shrink_enabled, bool, 0644);
>> +
>>   /*********************************
>>   * data structures
>>   **********************************/
>> @@ -947,6 +951,9 @@ static int zswap_shrink(void)
>>          struct zswap_pool *pool;
>>          int ret;
>>
>> +       if (!zswap_shrink_enabled)
>> +               return -EPERM;
>> +
>>          pool = zswap_pool_last_get();
>>          if (!pool)
>>                  return -ENOENT;
>> --
>> 2.7.4
>>