Re: Move PinBuffer and UnpinBuffer to atomics

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От Dilip Kumar
Тема Re: Move PinBuffer and UnpinBuffer to atomics
Дата
Msg-id CAFiTN-sb7y0Ud6XFvcv_U41W_eZHfZh69YBmcqutkfnO5-NMLA@mail.gmail.com
обсуждение исходный текст
Ответ на Re: Move PinBuffer and UnpinBuffer to atomics  (Alexander Korotkov <a.korotkov@postgrespro.ru>)
Ответы Re: Move PinBuffer and UnpinBuffer to atomics  (Dilip Kumar <dilipbalaut@gmail.com>)
Список pgsql-hackers
Дерево обсуждения

On Sun, Mar 20, 2016 at 4:10 AM, Alexander Korotkov <a.korotkov@postgrespro.ru> wrote:
Actually, we behave like old code and do such modifications without increasing number of atomic operations.  We can just calculate new value of state (including unset of BM_LOCKED flag) and write it to the buffer.  In this case we don't require more atomic operations.  However, it becomes not safe to use atomic decrement in UnpinBuffer().  We can use there loop of CAS which wait buffer header to be unlocked like PinBuffer() does.  I'm not sure if this affects multicore scalability, but I think it worth trying.

So, this idea is implemented in attached patch.  Please, try it for both regression on lower number of clients and scalability on large number of clients.


Good, seems like we have reduced some instructions, I will test it soon.
 
Other changes in this patch:
1) PinBuffer() and UnpinBuffer() use exponential backoff in spindealy like LockBufHdr() does.
2) Previous patch contained revert of ac1d7945f866b1928c2554c0f80fd52d7f977772.  This was not intentional.  No, it doesn't contains this revert.


Some other comments..

*************** ReadBuffer_common(SMgrRelation smgr, cha
*** 828,837 ****
  */
  do
  {
! LockBufHdr(bufHdr);
! Assert(bufHdr->flags & BM_VALID);
! bufHdr->flags &= ~BM_VALID;
! UnlockBufHdr(bufHdr);
  } while (!StartBufferIO(bufHdr, true));
  }
  }
--- 826,834 ----
  */
  do
  {
! uint32 state = LockBufHdr(bufHdr);
! state &= ~(BM_VALID | BM_LOCKED);
! pg_atomic_write_u32(&bufHdr->state, state);
  } while (!StartBufferIO(bufHdr, true));

Better Write some comment, about we clearing the BM_LOCKED from stage directly and need not to call UnlockBufHdr explicitly.
otherwise its confusing.



 
--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com

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