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SubjectRe: [PATCH] oprofile per-cpu buffer overrun
John Levon wrote:
> On Sun, Jan 25, 2004 at 08:07:01PM -0800, Andrew Morton wrote:
>
>
>>When implementing a circular buffer it is better to not constrain the head
>>and tail indices - just let them grow and wrap without bound. You only need
>>to bring them in-bounds when you actually use them to index the buffer.

neat!

> I'm not sure why that's better.

We win in increment_head/increment_tail:

static void increment_head(struct oprofile_cpu_buffer * b)
{
unsigned long new_head = b->head_pos + 1;
wmb();
-
if (new_head < (b->buffer_size))
b->head_pos = new_head;
-
else
-
b->head_pos = 0;
+
b->head_pos++;
}

for this added cost when accessing the buffer:

b->buffer[b->head & b->buffer_size_mask];

Modulo use is not worth but with buffer_size a power of 2
it's probably a win, I'll try and measure this later, not
urgent since the problem is fixed, I added it in our todo.

regards,
Phil

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