get_random_bytes() is expensive crypto-quality random numbers.
If we're just doing random backoff, prandom_u32() is plenty.
(Not to mention fetching 8 bytes of seed material only to
reduce it modulo 5000 is a huge waste.)
Also, convert timeouts to jiffies at compile time; convert
milliseconds to jiffies before picking a random number in the
range to take advantage of compile-time constant folding.
Signed-off-by: George Spelvin <[email protected]>
Cc: Mark Fasheh <[email protected]>
Cc: Joel Becker <[email protected]>
Cc: Joseph Qi <[email protected]>
Cc: [email protected]
---
fs/ocfs2/journal.c | 7 ++-----
1 file changed, 2 insertions(+), 5 deletions(-)
diff --git a/fs/ocfs2/journal.c b/fs/ocfs2/journal.c
index 68ba354cf3610..939a12e57fa8b 100644
--- a/fs/ocfs2/journal.c
+++ b/fs/ocfs2/journal.c
@@ -1884,11 +1884,8 @@ int ocfs2_mark_dead_nodes(struct ocfs2_super *osb)
*/
static inline unsigned long ocfs2_orphan_scan_timeout(void)
{
- unsigned long time;
-
- get_random_bytes(&time, sizeof(time));
- time = ORPHAN_SCAN_SCHEDULE_TIMEOUT + (time % 5000);
- return msecs_to_jiffies(time);
+ return msecs_to_jiffies(ORPHAN_SCAN_SCHEDULE_TIMEOUT) +
+ prandom_u32_max(5 * HZ);
}
/*
--
2.26.0
Sorry for the late reply since I might miss this mail.
On 2019/3/21 11:07, George Spelvin wrote:
> get_random_bytes() is expensive crypto-quality random numbers.
> If we're just doing random backoff, prandom_u32() is plenty.
>
> (Not to mention fetching 8 bytes of seed material only to
> reduce it modulo 5000 is a huge waste.)
>
> Also, convert timeouts to jiffies at compile time; convert
> milliseconds to jiffies before picking a random number in the
> range to take advantage of compile-time constant folding.
>
> Signed-off-by: George Spelvin <[email protected]>
> Cc: Mark Fasheh <[email protected]>
> Cc: Joel Becker <[email protected]>
> Cc: Joseph Qi <[email protected]>
> Cc: [email protected]
> ---
> fs/ocfs2/journal.c | 7 ++-----
> 1 file changed, 2 insertions(+), 5 deletions(-)
>
> diff --git a/fs/ocfs2/journal.c b/fs/ocfs2/journal.c
> index 68ba354cf3610..939a12e57fa8b 100644
> --- a/fs/ocfs2/journal.c
> +++ b/fs/ocfs2/journal.c
> @@ -1884,11 +1884,8 @@ int ocfs2_mark_dead_nodes(struct ocfs2_super *osb)
> */
> static inline unsigned long ocfs2_orphan_scan_timeout(void)
> {
> - unsigned long time;
> -
> - get_random_bytes(&time, sizeof(time));
> - time = ORPHAN_SCAN_SCHEDULE_TIMEOUT + (time % 5000);
> - return msecs_to_jiffies(time);
> + return msecs_to_jiffies(ORPHAN_SCAN_SCHEDULE_TIMEOUT) +
> + prandom_u32_max(5 * HZ);
Seems better include the prandom_u32_max() into msecs_to_jiffies()?
Thanks,
Joseph
> }
>
> /*
>
On Mon, Mar 30, 2020 at 08:09:33PM +0800, Joseph Qi wrote:
> Sorry for the late reply since I might miss this mail.
You're hardly late; I expect replies to dribble in for a week.
> On 2019/3/21 11:07, George Spelvin wrote:
>> diff --git a/fs/ocfs2/journal.c b/fs/ocfs2/journal.c
>> index 68ba354cf3610..939a12e57fa8b 100644
>> --- a/fs/ocfs2/journal.c
>> +++ b/fs/ocfs2/journal.c
>> @@ -1884,11 +1884,8 @@ int ocfs2_mark_dead_nodes(struct ocfs2_super *osb)
>> */
>> static inline unsigned long ocfs2_orphan_scan_timeout(void)
>> {
>> - unsigned long time;
>> -
>> - get_random_bytes(&time, sizeof(time));
>> - time = ORPHAN_SCAN_SCHEDULE_TIMEOUT + (time % 5000);
>> - return msecs_to_jiffies(time);
>> + return msecs_to_jiffies(ORPHAN_SCAN_SCHEDULE_TIMEOUT) +
>> + prandom_u32_max(5 * HZ);
>
> Seems better include the prandom_u32_max() into msecs_to_jiffies()?
What I'm trying to take advantage of here is constant propagation.
msecs_to_jiffies is zero cost (it's evaluated entirely at compile
time) if its argument is a compile-time constant. It's a function call
and a few instructions if its argument is variable.
msecs_to_jiffies(ORPHAN_SCAN_SCHEDULE_TIMEOUT + prandom_u32_max(5000))
would be forced to use the expensive version.
The compiler does't know, but *I* know, that msecs_to_jiffies() is a
linear function, and prandom_u32_max() is a sort-of linear function.
(It's a linear function for a given PRNG starting state, so each
individual call is linear, but multiple calls mess things up.)
Modulo a bit of rounding, we have:
msecs_to_jiffies(a + b) = msecs_to_jiffies(a) + msecs_to_jiffies(b)
msecs_to_jiffies(a) * b = msecs_to_jiffies(a * b)
prandom_u32_max(a) * b = prandom_u32_max(a * b)
prandom_u32_max(msecs_to_jiffies(a)) = msecs_to_jiffies(prandom_u32_max(a))
By doing the addition in jiffies rather than milliseconds, we get the
cheap code. It's not a huge big deal, but it's definitely smaller and
faster.
Admittedly, I happen to be using HZ = 300, which requires a multiply to
convert, and makes the resultant random numbers slightly non-uniform.
The default HZ = 250 makes it just a divide by 4, which is pretty simple.
(When HZ = 300, you get 1..3 ms -> 1 jiffy, 4..6 ms -> 2 jiffies, and
7..10 ms -> 3 jiffies. Multiples of 3 are 33% more likely to be chosen.)
It just seems silly and wasteful to pick a random number between 0 and
4999 (plus 30000), only to convert it to a random number between 0 and
1249 (plus 7500).
And if HZ = 2000 ever happens, the timeout won't be artificially limited
to integer milliseconds.
On 2020/3/31 00:34, George Spelvin wrote:
> On Mon, Mar 30, 2020 at 08:09:33PM +0800, Joseph Qi wrote:
>> Sorry for the late reply since I might miss this mail.
>
> You're hardly late; I expect replies to dribble in for a week.
>
>> On 2019/3/21 11:07, George Spelvin wrote:
>>> diff --git a/fs/ocfs2/journal.c b/fs/ocfs2/journal.c
>>> index 68ba354cf3610..939a12e57fa8b 100644
>>> --- a/fs/ocfs2/journal.c
>>> +++ b/fs/ocfs2/journal.c
>>> @@ -1884,11 +1884,8 @@ int ocfs2_mark_dead_nodes(struct ocfs2_super *osb)
>>> */
>>> static inline unsigned long ocfs2_orphan_scan_timeout(void)
>>> {
>>> - unsigned long time;
>>> -
>>> - get_random_bytes(&time, sizeof(time));
>>> - time = ORPHAN_SCAN_SCHEDULE_TIMEOUT + (time % 5000);
>>> - return msecs_to_jiffies(time);
>>> + return msecs_to_jiffies(ORPHAN_SCAN_SCHEDULE_TIMEOUT) +
>>> + prandom_u32_max(5 * HZ);
>>
>> Seems better include the prandom_u32_max() into msecs_to_jiffies()?
>
> What I'm trying to take advantage of here is constant propagation.
>
> msecs_to_jiffies is zero cost (it's evaluated entirely at compile
> time) if its argument is a compile-time constant. It's a function call
> and a few instructions if its argument is variable.
>
> msecs_to_jiffies(ORPHAN_SCAN_SCHEDULE_TIMEOUT + prandom_u32_max(5000))
> would be forced to use the expensive version.
>
> The compiler does't know, but *I* know, that msecs_to_jiffies() is a
> linear function, and prandom_u32_max() is a sort-of linear function.
>
> (It's a linear function for a given PRNG starting state, so each
> individual call is linear, but multiple calls mess things up.)
>
> Modulo a bit of rounding, we have:
>
> msecs_to_jiffies(a + b) = msecs_to_jiffies(a) + msecs_to_jiffies(b)
> msecs_to_jiffies(a) * b = msecs_to_jiffies(a * b)
> prandom_u32_max(a) * b = prandom_u32_max(a * b)
> prandom_u32_max(msecs_to_jiffies(a)) = msecs_to_jiffies(prandom_u32_max(a))
>
> By doing the addition in jiffies rather than milliseconds, we get the
> cheap code. It's not a huge big deal, but it's definitely smaller and
> faster.
>
> Admittedly, I happen to be using HZ = 300, which requires a multiply to
> convert, and makes the resultant random numbers slightly non-uniform.
> The default HZ = 250 makes it just a divide by 4, which is pretty simple.
>
> (When HZ = 300, you get 1..3 ms -> 1 jiffy, 4..6 ms -> 2 jiffies, and
> 7..10 ms -> 3 jiffies. Multiples of 3 are 33% more likely to be chosen.)
>
> It just seems silly and wasteful to pick a random number between 0 and
> 4999 (plus 30000), only to convert it to a random number between 0 and
> 1249 (plus 7500).
>
> And if HZ = 2000 ever happens, the timeout won't be artificially limited
> to integer milliseconds.
>
Thanks for the detail explanation.
Acked-by: Joseph Qi <[email protected]>