On Tue, Feb 5, 2019, 10:53 PM Xavi Hernandez <xhernan...@redhat.com wrote:

> On Fri, Feb 1, 2019 at 1:51 PM Xavi Hernandez <xhernan...@redhat.com>
> wrote:
>> On Fri, Feb 1, 2019 at 1:25 PM Poornima Gurusiddaiah <pguru...@redhat.com>
>> wrote:
>>> 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.
> I've run some simple tests with AIO enabled and results are not good. A
> simple dd takes >25% more time. Multiple parallel dd take 35% more time to
> complete.

Thank you. That is strange! Had few questions, what tests are you running
for measuring the io-threads performance(not particularly aoi)? is it dd
from multiple clients?


> Xavi
>> 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.
>> Xavi
>>> 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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