Re: [PATCH 04/10] Use percpu buffers

Nitin Gupta
2010-08-10T04:47:10Z

On 08/10/2010 12:27 AM, Pekka Enberg wrote:
> On Mon, Aug 9, 2010 at 8:26 PM, Nitin Gupta <ngupta@vflare.org> wrote:
>> @@ -303,38 +307,41 @@ static int zram_write(struct zram *zram, struct bio *bio)
>> zram_test_flag(zram, index, ZRAM_ZERO))
>> zram_free_page(zram, index);
>>
>> - mutex_lock(&zram->lock);
>> + preempt_disable();
>> + zbuffer = __get_cpu_var(compress_buffer);
>> + zworkmem = __get_cpu_var(compress_workmem);
>> + if (unlikely(!zbuffer || !zworkmem)) {
>> + preempt_enable();
>> + goto out;
>> + }
>
> The per-CPU buffer thing with this preempt_disable() trickery looks
> overkill to me. Most block device drivers seem to use mempool_alloc()
> for this sort of thing. Is there some reason you can't use that here?
>

Other block drivers are allocating relatively small structs using
mempool_alloc(). However, in case of zram, these buffers are quite
large (compress_workmem is 64K!). So, allocating them on every write
would probably be much slower than using a pre-allocated per-cpu buffer.

Thanks,
Nitin