Re: About user-level connection tracking mechanism: uselect()

On Wed, 25 Feb 2004, Xia Hongtao wrote:

> Had anyone heard about uselect()? uselect() is also an interface
> for web applications to improve the performance like epoll().It provide
> a user-level connection state tracking mechanism. Kernel and web
> applications share a piece of memory.There are some fd_sets in this
> shared memory.Each time the socket is ready to be read or write, the
> relative bit in shared memory will be set a flag.Most work of uselect()
> is just check these shared memory, without syscall and context switch,
> without fd_set copy.When there is no ready sockets,uselect() will block
> until any of them ready.

I have not heard of it before. Sounds interesting. Where can I find more
information about uselect?

Has there been any studies in how uselect positions itself in relation to
epoll/kpoll when the number of filedescriptors grows?

> The main problem to use this interface at squid-2.5 is: I do not know,
> in the original comm_select() loop, how many kinds of fd need to be
> checked by select()? Currently I see these: filesystem fds(for log
> files), TCP sockets,UDP sockets,pipes(for aio). My uselect currently can
> check state of sockets, and can be awaked by aio(thanks the help from
> Henrik).The next step I want to get rid of filesystem fds. Did I omit
> some ? Dose aio handle I/O of swap file ?

For filesystem fds you should just indicate readiness immediately. The
only reason why these appears in comm_select() in Squid is to break the
call stack.

> There are two way to handle filesystem fds:
>
> 1.Use aio to write log files,the main thread only handle network I/O(The
> squid-2.2 originally worked in this way).I think this will be the better
> method.Because if squid worked in this way, I need not do anything. :)

We are lazy and the performance difference is negliable as these are just
relatively low speed append writes which basically never blocks.

> 2.Always return ready for filesystem fds. When commSetSelect() be called
> to check the fs fds, I put them in another fd tables. Each time
> uselect() returns, it OR the result fd_set and that fd table.

This is how select() and poll() operates, and is what is expected by
Squid.

Regards
Henrik
Received on Wed Feb 25 2004 - 03:07:49 MST