netdev - Re: epoll, missed opportunity?

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Message-ID: <1442676118.29850.4.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Sat, 19 Sep 2015 08:21:58 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jonathan Marler <johnnymarler@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: epoll, missed opportunity?

On Fri, 2015-09-18 at 22:51 -0600, Jonathan Marler wrote:
> I'm curious why there wasn't another field added to the epoll_event
> struct for the application to store the descriptor's context. Any
> useful multi-plexing application will have a context that will need to
> be retrieved every time a descriptor needs to be serviced. Since the
> epoll api has no way of storing this context, it has to be looked up
> using the descriptor, which will take more time/memory as the number
> of descriptors increase. The memory saved from omitting this context
> can't be worth it since you'll have to allocate the memory in the
> application anyway, plus you're now adding the extra lookup.
> 
> This "lookup" problem has always existed in multi-plexed applications.
> It was impossible to fix with older polling interfaces, however, since
> epoll is stateful, it provides an opportunity to fix this problem by
> storing the descriptor context in epoll's "state". What was the reason
> for not doing this?  Was it an oversight or am I missing something?


typedef union epoll_data
{
  void *ptr;
  int fd;
  uint32_t u32;
  uint64_t u64;
} epoll_data_t;

struct epoll_event
{
  uint32_t events;      /* Epoll events */
  epoll_data_t data;    /* User data variable */
} __EPOLL_PACKED;



Application is free to use whatever is needed in poll_data_t

You can store a pointer to your own data (ptr)
Or a 32 bit cookie (u32)
Or a 64 bit cookie (u64)

(But is an union, you have to pick one of them)

Nothing forces you to use 'fd', kernel does not care.




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