On Wed, 13 Apr 2011, Andrew Morton wrote:
> Azurit reports large increases in system time after 2.6.36 when running
> Apache. It was bisected down to a892e2d7dcdfa6c76e6 ("vfs: use kmalloc()
> to allocate fdmem if possible").
>
> That patch caused the vfs to use kmalloc() for very large allocations and
> this is causing excessive work (and presumably excessive reclaim) within
> the page allocator.
>
> Fix it by falling back to vmalloc() earlier - when the allocation attempt
> would have been considered "costly" by reclaim.
>
> Reported-by: azurIt <azurit@xxxxxxxx>
> Cc: Changli Gao <xiaosuo@xxxxxxxxx>
> Cc: Americo Wang <xiyou.wangcong@xxxxxxxxx>
> Cc: Jiri Slaby <jslaby@xxxxxxx>
> Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> Cc: Mel Gorman <mel@xxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
>
> fs/file.c | 17 ++++++++++-------
> 1 file changed, 10 insertions(+), 7 deletions(-)
>
> diff -puN fs/file.c~a fs/file.c
> --- a/fs/file.c~a
> +++ a/fs/file.c
> @@ -39,14 +39,17 @@ int sysctl_nr_open_max = 1024 * 1024; /*
> */
> static DEFINE_PER_CPU(struct fdtable_defer, fdtable_defer_list);
>
> -static inline void *alloc_fdmem(unsigned int size)
> +static void *alloc_fdmem(unsigned int size)
> {
> - void *data;
> -
> - data = kmalloc(size, GFP_KERNEL|__GFP_NOWARN);
> - if (data != NULL)
> - return data;
> -
> + /*
> + * Very large allocations can stress page reclaim, so fall back to
> + * vmalloc() if the allocation size will be considered "large" by the VM.
> + */
> + if (size <= (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER) {
> + void *data = kmalloc(size, GFP_KERNEL|__GFP_NOWARN);
> + if (data != NULL)
> + return data;
> + }
> return vmalloc(size);
> }
>
It's a shame that we can't at least try kmalloc() with sufficiently large
sizes by doing something like
gfp_t flags = GFP_NOWAIT | __GFP_NOWARN;
if (size <= (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER))
flags |= GFP_KERNEL;
data = kmalloc(size, flags);
if (data)
return data;
return vmalloc(size);
which would at least attempt to use the slab allocator.
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