IO blocking under load

[O. Sinclair]

On 18/06/2016 01:35, Xen wrote:
> I wonder if anyone can answer the question why or how a system could be
> experiencing total IO blocking for basically all processes under certain
> conditions.
> 
> I am meaning to say that on this current system, I have experienced the
> majority of processes getting IO blocked (because they were reading from
> the disk what was seeing the "choke") for as long as 120 seconds or longer.
> 
> Meaning: the entire system becomes unresponsive (if this is the system
> disk) if any process actually does try to read from the disk. This IO
> blocking happens on a per-disk basis, it is not the entire system.
> 
> I have experienced it on a low SSD but I seem also to have experienced
> it on a regular HDD now. At least, while the SSD was connected, it was
> easy to reproduce it on the HDD as well.
> 
> The way that I reproduce it is by having e.g. a writing DD process and
> two reading find -> cat processes.
> 
> If you add a bunch of user programs (GUI programs) from KDE, it often
> enough to cause complete locking of the interface. If you go for a cup
> of tea, perhaps by the time you return it will have resolved itself, or
> not. Eventually the system resolves itself, but it can take a very long
> time.
> 
> Basically I was trying to add a SDD to my system, but it didn't take
> very long for this to start happening.
> 
> If anyone knows how or why this could happen, I would be much obliged if
> I had some pointers. For I am meaning to find out if it is due to the
> harddisk (SSD) or something else.
> 
> I have not tried changing IO scheduler yet. Regards.
> 
I can only give a very short answer: Yes I see it regularly since I
started using an SDD instead of HDD. System is 14.10 since any attempt
to go Plasma5 crashes totally.

Regards, Sinclair