Re: [PATCH] block: Improve shared tag set performance

[Bart Van Assche <bvanassche@acm.org>]

On 10/21/23 00:32, Yu Kuai wrote:
> Sorry for such huge delay, I was struggled on implementing a smoothly
> algorithm to borrow tags and return borrowed tags, and later I put this
> on ice and focus on other stuff.
> 
> I had an idea to implement a state machine, however, the amount of code
> was aggressive and I gave up. And later, I implemented a simple version,
> and I tested it in your case, 32 tags and 2 shared node, result looks
> good(see below), however, I'm not confident this can work well general.
> 
> Anyway, I'll send a new RFC verion for this, and please let me know if
> you still think this approch is unacceptable.
> 
> Thanks,
> Kuai
> 
> Test script:
> 
> [global]
> ioengine=libaio
> iodepth=2
> bs=4k
> direct=1
> rw=randrw
> group_reporting
> 
> [sda]
> numjobs=32
> filename=/dev/sda
> 
> [sdb]
> numjobs=1
> filename=/dev/sdb
> 
> Test result, by monitor new debugfs entry shared_tag_info:
> time    active        available
>      sda     sdb    sda    sdb
> 0    0    0    32    32
> 1    16    2    16    16    -> start fair sharing
> 2    19    2    20    16
> 3    24    2    24    16
> 4    26    2    28    16     -> borrow 32/8=4 tags each round
> 5    28    2    28    16    -> save at lease 4 tags for sdb

Hi Yu,

Thank you for having shared these results. What is the unit of the
numbers in the time column?

In the above I see that more tags are assigned to sda than to sdb
although I/O is being submitted to both LUNs. I think the current
algoritm defines fairness as dividing tags in a fair way across active
LUNs. Do the above results show that tags are divided per active job
instead of per active LUN? If so, I'm not sure that everyone will agree
that this is a fair way to distribute tags ...

Thanks,

Bart.