Hi,
On 2023-07-23 20:06:19 +0200, Oleksandr Natalenko wrote:
On neděle 23. července 2023 19:43:50 CEST Genes Lists wrote:
On 7/23/23 11:31, Jens Axboe wrote: ...
Just read the first one, but this is very much expected. It's now just correctly reflecting that one thread is waiting on IO. IO wait being 100% doesn't mean that one core is running 100% of the time, it just means it's WAITING on IO 100% of the time.
Seems reasonable thank you.
Question - do you expect the iowait to stay high for a freshly created mariadb doing nothing (as far as I can tell anyway) until process exited? Or Would you think it would drop in this case prior to the process exiting.
For example I tried the following - is the output what you expect?
Create a fresh mariab with no databases - monitor the core showing the iowaits with:
mpstat -P ALL 2 100# rm -f /var/lib/mysql/* # mariadb-install-db --user=mysql --basedir=/usr --datadir=/var/lib/mysql
# systemctl start mariadb (iowaits -> 100%)
# iotop -bo |grep maria (shows no output, iowait stays 100%)
(this persists until mariadb process exits)
# systemctl stop mariadb (iowait drops to 0%)
This is a visible userspace behaviour change with no changes in the userspace itself, so we cannot just ignore it. If for some reason this is how it should be now, how do we explain it to MariaDB devs to get this fixed?
Just to confirm I understand: Your concern is how it looks in mpstat, not performance or anything like that?
As far as I can tell, mariadb submits a bunch of IOs, which all have completed: ... mariadbd 438034 [000] 67593.094595: io_uring:io_uring_submit_req: ring 0xffff8887878ac800, req 0xffff888929df2400, user_data 0x55d5eaf29488, opcode READV, flags 0x0, sq_thread 0 mariadbd 438034 [000] 67593.094604: io_uring:io_uring_submit_req: ring 0xffff8887878ac800, req 0xffff888929df2500, user_data 0x55d5eaf29520, opcode READV, flags 0x0, sq_thread 0 mariadbd 438034 [000] 67593.094690: io_uring:io_uring_complete: ring 0xffff8887878ac800, req 0xffff888929df2400, user_data 0x55d5eaf29488, result 16384, cflags 0x0 extra1 0 extra2 0 mariadbd 438034 [000] 67593.094699: io_uring:io_uring_complete: ring 0xffff8887878ac800, req 0xffff888929df2500, user_data 0x55d5eaf29520, result 16384, cflags 0x0 extra1 0 extra2 0
Then waits for io_uring events: mariadbd 438032 [003] 67593.095282: io_uring:io_uring_cqring_wait: ring 0xffff8887878ac800, min_events 1
There won't be any completions without further IO being submitted.
io_uring could have logic to somehow report a different state in such a case (where there'll not be any completions before new IOs being submitted), but that'd likely not be free.
Greetings,
Andres Freund