On Fri, Feb 1, 2019 at 1:25 PM Poornima Gurusiddaiah <pguru...@redhat.com>

> Can the threads be categorised to do certain kinds of fops?

Could be, but creating multiple thread groups for different tasks is
generally bad because many times you end up with lots of idle threads which
waste resources and could increase contention. I think we should only
differentiate threads if it's absolutely necessary.

> Read/write affinitise to certain set of threads, the other metadata fops
> to other set of threads. So we limit the read/write threads and not the
> metadata threads? Also if aio is enabled in the backend the threads will
> not be blocked on disk IO right?

If we don't block the thread but we don't prevent more requests to go to
the disk, then we'll probably have the same problem. Anyway, I'll try to
run some tests with AIO to see if anything changes.

All this is based on the assumption that large number of parallel read
> writes make the disk perf bad but not the large number of dentry and
> metadata ops. Is that true?

It depends. If metadata is not cached, it's as bad as a read or write since
it requires a disk access (a clear example of this is the bad performance
of 'ls' in cold cache, which is basically metadata reads). In fact, cached
data reads are also very fast, and data writes could go to the cache and be
updated later in background, so I think the important point is if things
are cached or not, instead of if they are data or metadata. Since we don't
have this information from the user side, it's hard to tell what's better.
My opinion is that we shouldn't differentiate requests of data/metadata. If
metadata requests happen to be faster, then that thread will be able to
handle other requests immediately, which seems good enough.

However there's one thing that I would do. I would differentiate reads
(data or metadata) from writes. Normally writes come from cached
information that is flushed to disk at some point, so this normally happens
in the background. But reads tend to be in foreground, meaning that someone
(user or application) is waiting for it. So I would give preference to
reads over writes. To do so effectively, we need to not saturate the
backend, otherwise when we need to send a read, it will still need to wait
for all pending requests to complete. If disks are not saturated, we can
have the answer to the read quite fast, and then continue processing the
remaining writes.

Anyway, I may be wrong, since all these things depend on too many factors.
I haven't done any specific tests about this. It's more like a
brainstorming. As soon as I can I would like to experiment with this and
get some empirical data.


> Thanks,
> Poornima
> On Fri, Feb 1, 2019, 5:34 PM Emmanuel Dreyfus <m...@netbsd.org wrote:
>> On Thu, Jan 31, 2019 at 10:53:48PM -0800, Vijay Bellur wrote:
>> > Perhaps we could throttle both aspects - number of I/O requests per disk
>> While there it would be nice to detect and report  a disk with lower than
>> peer performance: that happen sometimes when a disk is dying, and last
>> time I was hit by that performance problem, I had a hard time finding
>> the culprit.
>> --
>> Emmanuel Dreyfus
>> m...@netbsd.org
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