With regard to ‘db file parallel read’ with sync IO, there seems to be a
difference between Solaris x64 and Linux. On Solaris I’m seeing a bunch of
serial pread calls (as opposed to preadv on Linux). The sum of their durations
seems to be contained in the wait event time. My observation refers to a
non-multitenant 12.1 instance running on Solaris 11.4. Therefore, the async IO
would bring a huge benefit, provided there’s enough bandwidth to handle
parallel IO calls.
Best regards,
Nenad
https://nenadnoveljic.com/blog
From: Frits Hoogland <frits.hoogland@xxxxxxxxx>
Sent: Samstag, 29. Februar 2020 12:35
To: Noveljic Nenad <nenad.noveljic@xxxxxxxxxxxx>
Cc: Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx>; oracle-l@xxxxxxxxxxxxx
Subject: Re: parallel recovery slaves waiting on undo reads
Sadly, wait event timing can be influenced by database parameters, and recently
I found that multi tenant changed the way the timing was done too.
Wait events typically (but not always!) time system call(s), for which the wait
event time sometimes is an indication of performance in another layer in the
application stack.
I often use wait events to talk to for example storage admins about
performance. Therefore, it’s critical that the wait event timings do correlate
with timings of the admins of the other layer, so we can work together. This is
one of the most important reasons I study wait events to the level that I do;
so I understand what the timing incorporates, and therefore can explain that to
for example the storage admin.
Last time I checked, the db file parallel read wait event timing for
asynchronous IO looked like this:
1. io_submit (multiple IOs via an iocb struct, see:
http://man7.org/linux/man-pages/man2/io_submit.2.html)
2. start wait event
3. io_getevents (blocking; wait for all IOs to finish)
4. end wait event
So not the total IO time is timed (although a very little part of it isn’t),
and indeed if you disregard the small part that isn’t timed, it’s the timing of
the slowest IO of all IOs that this wait event shows.
For synchronous IO, preadv() performs the same function, but with submission
and waiting combined. All the IOs are submitted via an iovec
(https://linux.die.net/man/2/preadv) using a single system call. The timing of
this system call is obvious:
1. start wait event
2. preadv
3. end wait event
I can’t find a definitive source that tells me how preadv is implemented on
linux. I would assume that linux is prepared for modern IO and does not assume
it’s operating on a single disk and therefore performing the different IOs
serially, but as I said, I would love to be pointed to the kernel source where
the vector read is performed to validate it being serial or parallel.
So for pread, I hope it’s the maximum time of the slowest IO, but it could be
the sum of all individual IO times (serial).
I recently studied log file parallel write (again) for a conference is poland.
Much to my surprise I found that the log file parallel write timing was done in
the following way
1. io_submit
2. io_getevents (non-blocking; if all IOs are found goto 6)
3. start wait event
4. io_getevents (blocking)
5. end wait event
6. done
In other words: if the IO subsystem is fast enough, the wait event does not
occur at all.
This is consistent with what I found years ago with Oracle’s asynchronous
direct path read implementation.
However, this was with multi-tenancy turned on. With it turned off, the timing
became:
1. start wait event
2. io_submit
3. io_getevents
4. end wait event
I am surprised that multi-tenancy has this massive change in timing
implementation. Of course the wait event timing (the latency) does not change
that much, and both essentially give the IO time of the longest taking IO.
In the light of new technologies like persistent writable memory (“pmem”) I can
see this making sense: if the IO is nearly instantaneous, assume it is, and
only start the time accounting (alias wait event) if it turns out it isn’t.
Frits Hoogland
http://fritshoogland.wordpress.com<http://fritshoogland.wordpress.com/>
frits.hoogland@xxxxxxxxx<mailto:frits.hoogland@xxxxxxxxx>
Mobile: +31 6 14180860
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