linux-kernel - Re: [PATCH] epoll: trim epitem by one cache line on x86

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Message-ID: <20130307103240.GA24132@dcvr.yhbt.net>
Date:	Thu, 7 Mar 2013 10:32:40 +0000
From:	Eric Wong <normalperson@...t.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Davide Libenzi <davidel@...ilserver.org>,
	Al Viro <viro@...IV.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] epoll: trim epitem by one cache line on x86_64

Andrew Morton <akpm@...ux-foundation.org> wrote:
> It's going to be hard to maintain this - someone will change something
> sometime and break it.  I suppose we could add a runtime check if we
> cared enough.  Adding a big fat comment to struct epitem might help.

Thanks for looking at this patch.  I'll send a patch with a comment
about keeping epitem size in check.  Also, would adding (with comments):

	BUILD_BUG_ON(sizeof(struct epitem) > 128);

...be too heavy-handed?  I used that in my testing.  I'll check for:
sizeof(void *) <= 8 too; in case 128-bit machines appear...

> I don't see much additional room to be saved.  We could probably remove
> epitem.nwait, but that wouldn't actually save anything because nwait
> nestles with ffd.fd.

If we remove nwait, we can move epoll_event up and have event.events
tucked in there.  I have more and more depending on epoll, so I'll be
around to comment on future epoll changes as they come up.

> I tested your patch on powerpc and it reduced sizeof(epitem) from 136
> to 128 for that arch as well, so I suggest we run with
> 
> --- a/fs/eventpoll.c~epoll-trim-epitem-by-one-cache-line-on-x86_64-fix
> +++ a/fs/eventpoll.c
> @@ -105,7 +105,7 @@
>  struct epoll_filefd {
>  	struct file *file;
>  	int fd;
> -} EPOLL_PACKED;
> +} __packed;

Thanks for testing on ppc.  Looks good to me.  For what it's worth:
Acked-by: Eric Wong <normalperson@...t.net>
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