[systemd-devel] Improve boot-time of systemd

[fykcee1 at gmail.com]

2011/3/28 Lennart Poettering <lennart at poettering.net>:
> On Sun, 20.03.11 05:28, fykcee1 at gmail.com (fykcee1 at gmail.com) wrote:
>> Current readahead implementation has some problems:
>> 1. It can't separate *real* block read requests from all read
>> requests(which includes more blocks read by the kernel's readahead
>> logic)
>
> Shouldn't make a big difference, since on replay we turn off additional
> kernel-side readahead.
>
> However, it is true that the file will only ever increase, never
> decrease in size.
For collect, it can't filter out:
1. Kernel-side readahead, whether the readahead is initiated by
kernel(when no /.readhead data), or the replay process.
2. Written blocks of files(opened as "r+", "w+", "a"). The written
blocks resides at memory when boot time.

IMHO, the kernel lacks some APIs to notify each *real* read requests.
e.g, It can be done by tracking each read syscall (mmap seems not easy
to handle, though).

>
>> 2. It just gives advices for how to do kernel's readahead, causes the
>> first read of a fille to spend more time.
>
> Hmm?
posix_fadvise(...) may make each read do more readahead(more than the
kernel guess way), thus spend more time. e.g.
* When no replay, someone reads A part of file X --> do some job -->
reads B part of file X.
* When replay, both A and B parts of file X are read in one time, thus
more I/O usage. Other services may spend more time waiting for
I/O.(This can be observed from bootchart diagram)

BTW, does posix_fadvise apply globally or just for the process which calls it?

>
> We do that too. We use "idle" on SSD, and "realtime" on HDD.
Why "realtime" on HDD?

BTW, According to test, the "idle" is not really *idle*, see the attachment.
That means 'replay' will always impact other one's I/O. For 'replay'
in idle I/O class on HDD, other one's I/O performance will reduce by
half, according to the test.



-- 
Regards,
- cee1
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