Difference between IOWait(R) and IOWait(R)(041) ?

[nelis]

Hi!

As the subject says I'm curious to know the difference between these two states as seen in the Task Status of Kernel Threads task manager. I couldn't find anything in the documentation either. It appears they are both awaiting results from the database(reads) but what is the difference ?

I'm gathering that the ":-(" in the status description for IOWait(R)(041) is not a good sign

Thanks.

Regards,

Nelis

[markus_doehr2 · Accepted solution]

I think the 041 is a indication, that two UKTs are reading on the same Btree - same table and one is waiting until the other is finished.

I may be wrong but I believe it has something to do with concurrency.

Markus

[nelis · Accepted solution]

Thanks Marcus, if I look at our current scenario that makes sense.

I'll see if anyone can confirm this before closing thread.

Regards,

Nelis

[lbreddemann · Accepted solution]

> I think the 041 is a indication, that two UKTs are reading on the same Btree - same table and one is waiting until the other is finished.

>

> I may be wrong but I believe it has something to do with concurrency.

Pretty close!

In fact, suspend reason "IOWait(R)(041) b13get_node: await read" means:

1. the requested page is not found in the cache

and

2. another task has already triggered that this page will be read from disk into cache.

From application point of view, this can mean totally different things - same data is accessed in parallel, different data is accessed in parallel. This totally depends on what kind of page is looked for now.

E.g. even if two tasks want to read the same page it can be about completely different data (think about root- or index-pages).

regards,

Lars

[markus_doehr2 · Accepted solution]

> Pretty close!

>

> In fact, suspend reason "IOWait(R)(041) b13get_node: await read" means:

> 1. the requested page is not found in the cache

> and

> 2. another task has already triggered that this page will be read from disk into cache.

Thanx for clarifying!

Markus

[nelis · Accepted solution]

Thanks Lars!

Regards,

Nelis