On 10/26/2012 04:02 PM, Fengguang Wu wrote:
On Fri, Oct 26, 2012 at 03:47:19PM +0800, Ni zhan Chen wrote:
On 10/26/2012 03:36 PM, Fengguang Wu wrote:
On Fri, Oct 26, 2012 at 03:19:57PM +0800, Ni zhan Chen wrote:
On 10/26/2012 03:09 PM, Fengguang Wu wrote:
On Fri, Oct 26, 2012 at 03:03:12PM +0800, Ni zhan Chen wrote:
On 10/26/2012 02:58 PM, Fengguang Wu wrote:
static void shrink_readahead_size_eio(struct file *filp,
struct file_ra_state *ra)
{
- ra->ra_pages /= 4;
+ spin_lock(&filp->f_lock);
+ filp->f_mode |= FMODE_RANDOM;
+ spin_unlock(&filp->f_lock);
As the example in comment above this function, the read maybe still
sequential, and it will waste IO bandwith if modify to FMODE_RANDOM
directly.
Yes immediately disabling readahead may hurt IO performance, the
original '/ 4' may perform better when there are only 1-3 IO errors
encountered.
Hi Fengguang,
Why the number should be 1-3?
The original behavior is '/= 4' on each error.
After 1 errors, readahead size will be shrinked by 1/4
After 2 errors, readahead size will be shrinked by 1/16
After 3 errors, readahead size will be shrinked by 1/64
After 4 errors, readahead size will be effectively 0 (disabled)
But from function shrink_readahead_size_eio and its caller
filemap_fault I can't find the behavior you mentioned. How you
figure out it?
It's this line in shrink_readahead_size_eio():
ra->ra_pages /= 4;
Yeah, I mean why the 4th readahead size will be 0(disabled)? What's
the original value of ra->ra_pages? How can guarantee the 4th shrink
readahead size can be 0?
Ah OK, I'm talking about the typical case. The default readahead size
is 128k, which will become 0 after / 256. The reasonable good ra size
for hard disks is 1MB=256pages, which also becomes 1page after 4 errors.
Then why default size is not set to reasonable size?
Regards,
Chen
Thanks,
Fengguang
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