Messages in this thread | | | From | NeilBrown <> | Date | Tue, 06 Aug 2019 09:46:27 +1000 | Subject | Re: Bisected: Kernel 4.14 + has 3 times higher write IO latency than Kernel 4.4 with raid1 |
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On Mon, Aug 05 2019, Jinpu Wang wrote:
> Hi Neil, > > For the md higher write IO latency problem, I bisected it to these commits: > > 4ad23a97 MD: use per-cpu counter for writes_pending > 210f7cd percpu-refcount: support synchronous switch to atomic mode. > > Do you maybe have an idea? How can we fix it?
Hmmm.... not sure.
My guess is that the set_in_sync() call from md_check_recovery() is taking a long time, and is being called too often.
Could you try two experiments please.
1/ set /sys/block/md0/md/safe_mode_delay to 20 or more. It defaults to about 0.2.
2/ comment out the call the set_in_sync() in md_check_recovery().
Then run the least separately after each of these changes.
I the second one makes a difference, I'd like to know how often it gets called - and why. The test if ( ! ( (mddev->sb_flags & ~ (1<<MD_SB_CHANGE_PENDING)) || test_bit(MD_RECOVERY_NEEDED, &mddev->recovery) || test_bit(MD_RECOVERY_DONE, &mddev->recovery) || (mddev->external == 0 && mddev->safemode == 1) || (mddev->safemode == 2 && !mddev->in_sync && mddev->recovery_cp == MaxSector) )) return;
should normally return when doing lots of IO - I'd like to know which condition causes it to not return.
Thanks, NeilBrown [unhandled content-type:application/pgp-signature] | |