Re: [RFC PATCH] Fix Readahead stalling by plugged device queues

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 





Wu Fengguang wrote:
[...]
Christian, did you notice this commit for 2.6.33?

commit 65a80b4c61f5b5f6eb0f5669c8fb120893bfb388
[...]

I didn't see that particular one, due to the fact that whatever the result is it needs to work .32

Anyway I'll test it tomorrow and if that already accepted one fixes my issue as well I'll recommend distros older than 2.6.33 picking that one up in their on top patches.


It should at least improve performance between .32 and .33, because
once two readahead requests are merged into one single IO request,
the PageUptodate() will be true at next readahead, and hence
blk_run_backing_dev() get called to break out of the suboptimal
situation.

As you saw from my blktrace thats already the case without that patch.
Once the second readahead comes in and merged it gets unplugged in 2.6.32 too - but still that is bad behavior as it denies my things like 68% throughput improvement :-).


Your patch does reduce the possible readahead submit latency to 0.

yeah and I think/hope that is fine, because as I stated:
- low utilized disk -> not an issue
- high utilized disk -> unplug is an noop

At least personally I consider a case where merging of a readahead window with anything except its own sibling very rare - and therefore fair to unplug after and RA is submitted.

Is your workload a simple dd on a single disk? If so, it sounds like
something illogical hidden in the block layer.

It might still be illogical hidden as e.g. 2.6.27 unplugged after the first readahead as well :-) But no my load is iozone running with different numbers of processes with one disk per process. That neatly resembles e.g. nightly backup jobs which tend to take longer and longer in all time increasing customer scenarios. Such an improvement might banish the backups back to the night were they belong :-)

Thanks,
Fengguang

--

Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, System z Linux Performance

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxxx  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>

[Index of Archives]     [Linux ARM Kernel]     [Linux ARM]     [Linux Omap]     [Fedora ARM]     [IETF Annouce]     [Bugtraq]     [Linux]     [Linux OMAP]     [Linux MIPS]     [ECOS]     [Asterisk Internet PBX]     [Linux API]