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x86, olpc: Speed up device tree creation during boot

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Calling alloc_bootmem() for tiny chunks of memory over and over is really
slow; on an XO-1, it caused the time between when the kernel started
booting and when the display came alive (post-lxfb probe) to increase
to 44s.  This patch optimizes the prom_early_alloc function by
calling alloc_bootmem for 4k-sized blocks of memory, and handing out
chunks of that to callers.  With this patch, the time between kernel load
and display initialization decreased to 23s.  If there's a better way to
do this early in the boot process, please let me know.

(Note: increasing the chunk size to 16k didn't noticably affect boot time,
and wasted 9k.)

v4: clarify comment, requested by hpa
v3: fix wasted memory buglet found by Milton Miller, and style fix.
v2: reorder prom_early_alloc as suggested by Grant.

Signed-off-by: Andres Salomon <dilinger@queued.net>
LKML-Reference: <20101129153951.74202a84@queued.net>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
This commit is contained in:
Andres Salomon 2010-11-29 23:39:51 +00:00 committed by H. Peter Anvin
parent c10d1e260f
commit b5318d302f
1 changed files with 22 additions and 4 deletions
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26
arch/x86/platform/olpc/olpc_dt.c
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@ -126,14 +126,32 @@ static unsigned int prom_early_allocated __initdata;
void * __init prom_early_alloc(unsigned long size)
{
static u8 *mem;
static size_t free_mem;
void *res;
res = alloc_bootmem(size);
if (res)
memset(res, 0, size);
if (free_mem < size) {
const size_t chunk_size = max(PAGE_SIZE, size);
prom_early_allocated += size;
/*
* To mimimize the number of allocations, grab at least
* PAGE_SIZE of memory (that's an arbitrary choice that's
* fast enough on the platforms we care about while minimizing
* wasted bootmem) and hand off chunks of it to callers.
*/
res = alloc_bootmem(chunk_size);
if (!res)
return NULL;
prom_early_allocated += chunk_size;
memset(res, 0, chunk_size);
free_mem = chunk_size;
mem = res;
}
/* allocate from the local cache */
free_mem -= size;
res = mem;
mem += size;
return res;
}