linux-kernel - Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

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Date:	Mon, 26 Feb 2007 12:02:51 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Evgeniy Polyakov <johnpol@....mipt.ru>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@....com.au>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ulrich Drepper <drepper@...hat.com>,
	Zach Brown <zach.brown@...cle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

On Mon, 26 Feb 2007, Ingo Molnar wrote:

> 
> * Ingo Molnar <mingo@...e.hu> wrote:
> 
> > please also try evserver_epoll_threadlet.c that i've attached below - 
> > it uses epoll as the main event mechanism but does threadlets for 
> > request handling.
> 
> find updated code below - your evserver_epoll.c spuriously missed event 
> edges - so i changed it back to level-triggered. While that is not as 
> fast as edge-triggered, it does not result in spurious hangs and 
> workflow 'hickups' during the test.
> 
> Could this be the reason why in your testing kevents outperformed epoll?

This is how I handle a read (write/accept/connect, same thing) inside 
coronet (coroutine+epoll async library - http://www.xmailserver.org/coronet-lib.html ).


static int conet_read_ll(struct sk_conn *conn, char *buf, int nbyte) {
        int n;
        
        while ((n = read(conn->sfd, buf, nbyte)) < 0) {
                if (errno == EINTR)
                        continue;
                if (errno != EAGAIN && errno != EWOULDBLOCK)
                        return -1;
                if (!(conn->events & EPOLLIN)) {
                        conn->events = EPOLLIN;
                        if (conet_mod_conn(conn, conn->events) < 0)
                                return -1;
                }
                if (conet_yield(conn) < 0)
                        return -1;
        }
                        
        return n;
}

I use EPOLLET and, you don't change the interest set until you actually 
get an EAGAIN. *Many* read/write mode changes in the usage will simply 
happen w/out an epoll_ctl() needed. The conet_mod_conn() function does the 
actual epoll_ctl() and add EPOLLET to the specified event set. The 
conet_yield() function end up calling the libpcl's co_resume(), that is 
basically a switch-to-next-coroutine-until-fd-becomes-ready (maps 
directly to a swapcontext).
That cuts 50+% of the epoll_ctl(EPOLL_CTL_MOD).




- Davide


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