Re: [sqlite] Difference between min and max time, especially for sys

[Cecil Westerhof]

2017-02-16 19:31 GMT+01:00 Jens Alfke :

>
> > On Feb 15, 2017, at 11:47 AM, Cecil Westerhof 
> wrote:
> >
> > But the difference between sys can be almost a factor twenty. What seems
> very big to me.
>
> Yup. There is a huge speed difference between the kernel going to the
> disk/SSD to read from a file, vs. the kernel reading the data out of its
> buffer cache. That’s why the kernel has buffer caches :)
>
> (And those disk reads can be hugely variable. A hard disk can spend 10ms
> or more seeking, and any medium has bandwidth limits, so other processes’
> I/O can greatly slow down your own. I’ve seen filesystem slowdowns of 20x
> or more shortly after a user logs in when dozens of processes are launching
> at once.)
>

​Well user/sys is mostly around 10, so it does not have a to big influence
on the measurements. ;-)

-- 
Cecil Westerhof
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Re: [sqlite] Difference between min and max time, especially for sys

[Jens Alfke]

> On Feb 15, 2017, at 11:47 AM, Cecil Westerhof  wrote:
> 
> But the difference between sys can be almost a factor twenty. What seems very 
> big to me.

Yup. There is a huge speed difference between the kernel going to the disk/SSD 
to read from a file, vs. the kernel reading the data out of its buffer cache. 
That’s why the kernel has buffer caches :)

(And those disk reads can be hugely variable. A hard disk can spend 10ms or 
more seeking, and any medium has bandwidth limits, so other processes’ I/O can 
greatly slow down your own. I’ve seen filesystem slowdowns of 20x or more 
shortly after a user logs in when dozens of processes are launching at once.)

—Jens
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Re: [sqlite] Difference between min and max time, especially for sys

[Simon Slavin]

On 15 Feb 2017, at 7:47pm, Cecil Westerhof  wrote:

> So the difference between minimum and maximum is about a factor two. What
> seems reasonable to me. But the difference between sys can be almost a
> factor twenty. What seems very big to me.

Caching affects sys.  Might be difference between time taken to fetch from disk 
and time taken when already in memory.

Simon.
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