I now read the docs for  the AIX pollset API, and it does not seem an easy 
thing to use for the threadpool.
https://www.ibm.com/support/knowledgecenter/en/ssw_aix_71/p_bostechref/pollset.html

The main hurdle is that there is no “one-shot” behaviour, so you will need to 
pollset_ctl(PS_DELETE) each returned descriptor, and 
pollset_ctl(PS_ADD) it later. That would be a good workaround, but  very 
surprising behaviour of the pollset is that pollset_ctl() waits for  
pollset_poll() to complete, which makes it not better than a normal poll() 
really. I’ve no idea what IBM folks were thinking of when they designed this 
API.

As for  normal poll(2) 
The problem with it, is that if one does not immediately drain data from 
readable socket, poll() will busy-loop returning the same fds again and again. 
It is possible to fix busy loop, by clearing pollfd.events flag, but then again 
we need reset pollfd.events soon, after threadpool_process_request(). And for 
that, we’d need to somehow interrupt poll() , modify the fds array, issue 
poll() again. There will be a lot of interrupts, I’m afraid.  I foresee a busy 
loop either way, which makes poll(2) not a very good option. Unless you  can 
figure out how to drain all data from socket immediately after it becomes 
readable.

IOCP sounds really like a better API, in case you can use it.

From: Vladislav Vaintroub
Sent: Tuesday, 17 September 2019 15:20
To: GUESNET, ETIENNE (ext); maria-developers@lists.launchpad.net
Subject: RE: [Maria-developers] Implementation of Threadpool

Hi Etienne,
The reason why there is no poll/select implementation is that the systems that 
we support all have something better then poll/select, which can be used 
instead. So yes, “no interest” would fit. The main factor is of course that we 
have no access  to those commercial Unix distributions that have neither of  
IOCP,epoll, kevent or ports 

The  notification property that we want to have here is that, once there is 
some data on a socket coming from client, the socket is returned, and is taken 
out of the “poll set”. Socket returns to the “poll set” once command(e.g SQL 
query , or anything else that client-server protocol understands) is fully 
processed. We’d like to avoid multiple notifications on a socket,  until client 
command is fully processed, otherwise different threads from the pool will try 
to process client’s command at the same time. 
This is the only thing we need, and this is something that poll/select do not 
provide.

As for Solaris ports – 
existing Solaris implementation is based on ports, and as far as I could test, 
it had this one-shot behaviour, which is that we need, I.e port_get() only 
returns a single event, and then there must be a new port_associate() to 
reenable the socket/return it to “poll-set”. The documentation confirms this 
observation
“Objects of type PORT_SOURCE_FD are file descriptors. The event types for 
PORT_SOURCE_FD objects are described in poll(2). At most one event notification 
will be generated per associated file descriptor. For example, if a file 
descriptor is associated with a port for the POLLRDNORM event and data is 
available on the file descriptor at the time the port_associate() function is 
called, an event is immediately sent to the port. If data is not yet available, 
one event is sent to the port when data first becomes available.”

I read somewhere IOCP would exist on AIX, so I hoped existing Windows code in 
threadpol_generic.cc would be of some help, if someone eventually ports that to 
AIX . Although, admittedly Windows code,  uses a trick to reading zero bytes in 
ReadFile/WSARecv, which effectively translates asynchronous IO to “poll-like” 
notification modes.

From: GUESNET, ETIENNE (ext)
Sent: Tuesday, 17 September 2019 14:13
To: maria-developers@lists.launchpad.net
Subject: [Maria-developers] Implementation of Threadpool

Hi,
I am implementing a threadpool system to AIX. The AIX equivalent of epoll / 
kqueue on AIX is pollset (and IOCP, but partial implementation only). However, 
pollset has only a level-trigger mode and MariaDB needs edge-trigger (see 
comments of sql/threadpoll_generic.h file). Adding a pollset support in MariaDB 
would be difficult, and probably not so efficient, as we need to simulate the 
edge-trigger behavior.
Obviously, AIX has poll and select support. MariaDB has not. Is there a reason 
to don’t implement threadpoll through poll or select? No interest? Performance 
issues?
MariaDB currently works on AIX without threadpool; in term of efficiency, do 
you know what can be obtained using threadpool with poll/select or a more 
modern solution?
As far I know, SunOS/Solaris/Illumos threadpoll system (called “port”) is also 
level-trigger only, but I do not find specific functions to manage this.
Thanks!
Etienne Guesnet.


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