RE: Question about ufs_bsg

[Avri Altman]

Hi,

> Hi,
> 
> ufs_bsg was introduced nearly three years ago and it allocates its own request
> queue.
> I faced a sytmpom with this and want to ask something about it.
> 
> That is, sometimes queue depth for ufs is limited to half of the its maximum
> value
> even in a situation with many IO requests from filesystem.
This is interesting indeed. Before going further with investigating this,
Could you share some more details on your setup:
The bsg node it creates was originally meant to convey a single query request via SG_IO ioctl,
Which is blocking.
 - How do you create many IO requests queueing on that request queue?
 - command upiu is not implemented, are all those IOs are query requests?

Thanks,
Avri

> It turned out that it only occurs when a query is being processed at the same
> time.
> Regarding my tracing, when the query process starts, users for the hctx that
> represents
> a ufs host increase to two and with this, some pathes calling 'hctx_may_queue'
> function in blk-mq seems to throttle dispatches, technically with 16 because the
> number of
> ufs slots (32 in my case) is dividend by two (users).
> 
> I found that it happened when a query for write booster is processed
> because write booster only turns on in some conditions in my base that is
> different
> from kernel mainline. But when an exceptional event or others that could lead
> to a query occurs,
> it can happen even in mainline.
> 
> I think the throttling is a little bit excessive,
> so the question: is there any way to assign queue depth per user on an
> asymmetric basis?
> 
> Thanks.
> Kiwoong Kim
>