Message-ID: <2c449944-443d-b093-224d-73b21f19d456@gmail.com>
Date: Thu, 16 Mar 2023 12:50:35 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0)
 Gecko/20100101 Thunderbird/102.8.0
Subject: Re: Speed-up shared buffers prewarming
Content-Language: en-US
To: Matthias van de Meent <boekewurm+postgres@gmail.com>,
 Konstantin Knizhnik <knizhnik@garret.ru>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
References: <b99d8b95-caed-9e39-4fd6-2cbd47224759@garret.ru>
 <CAEze2WjJckQm0SfRuMXs8fMYtgpS++bkHEGkcmqNVgnrZte-5g@mail.gmail.com>
From: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
In-Reply-To: <CAEze2WjJckQm0SfRuMXs8fMYtgpS++bkHEGkcmqNVgnrZte-5g@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Archived-At: <https://www.postgresql.org/message-id/2c449944-443d-b093-224d-73b21f19d456%40gmail.com>
Precedence: bulk

Hi,

On 3/15/23 10:40 PM, Matthias van de Meent wrote:
> On Wed, 15 Mar 2023 at 21:38, Konstantin Knizhnik <knizhnik@garret.ru> wrote:
>>
>> Hi hackers,
>>
>> It is well known fact that queries using sequential scan can not be used to prewarm cache, because them are using ring buffer
>> even if shared buffers are almost empty.
>> I have searched hackers archive but failed to find any discussion about it.
>> What are the drawbacks of using free buffers even with BAM_BULKREAD strategy?
>> I mean the following trivial patch:
>>
>> diff --git a/src/backend/storage/buffer/freelist.c b/src/backend/storage/buffer/freelist.c
>> index 6be80476db..243335d0e4 100644
>> --- a/src/backend/storage/buffer/freelist.c
>> +++ b/src/backend/storage/buffer/freelist.c
>> @@ -208,8 +208,15 @@ StrategyGetBuffer(BufferAccessStrategy strategy, uint32 *buf_state)
>>          /*
>>           * If given a strategy object, see whether it can select a buffer. We
>>           * assume strategy objects don't need buffer_strategy_lock.
>>           */
>> -       if (strategy != NULL)
>> +       if (strategy != NULL && StrategyControl->firstFreeBuffer < 0)
>>          {
>>                  buf = GetBufferFromRing(strategy, buf_state);
>>                  if (buf != NULL)
>>
>> So if there are free buffers, then use normal buffer allocation instead of GetBufferFromRing.
> 
> Yes. As seen in [1], ring buffers aren't all that great in some cases,
> and I think this is one. Buffer allocation should always make use of
> the available resources, so that it doesn't take O(N/ring_size) scans
> on a table to fill the buffers if that seqscan is the only workload of
> the system.

Agree but then what do you think about also paying special attention to those buffers when eviction needs to be done?

Those buffers are "usually" needed briefly, so something like being able to distinguish them and be more aggressive regarding their eviction.

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com