On Mon, 4 Mar 2013 11:29:41 +0000 Eric Wong <normalperson@xxxxxxxx> wrote:
> It is common for epoll users to have thousands of epitems, so saving a
> cache line on every allocation leads to large memory savings.
>
> Since epitem allocations are cache-aligned, reducing sizeof(struct
> epitem) from 136 bytes to 128 bytes will allow it to squeeze under a
> cache line boundary on x86_64.
>
> >From /sys/kernel/slab/eventpoll_epi, I see the following changes on my
> x86_64 Core2 Duo (which has 64-byte cache alignment):
>
> object_size : 192 => 128
> objs_per_slab: 21 => 32
>
> I have no access to other 64-bit machines, so I am limiting this to
> x86_64-only with EPOLL_PACKED instead of __attribute__((packed))
>
> ...
>
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -107,7 +107,7 @@
> struct epoll_filefd {
> struct file *file;
> int fd;
> -};
> +} EPOLL_PACKED;
>
> /*
> * Structure used to track possible nested calls, for too deep recursions
Yes, I see the same numbers on my gcc, x86_64 allmodconfig.
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.
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.
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;
/*
* Structure used to track possible nested calls, for too deep recursions
_
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