[LU-1147] Reduce CPU overhead and performance degradation with larger striped files while reading uncached data in 1.8

• Type: Improvement
• Resolution: Won't Fix
• Priority: Minor
• Affects Version/s: Lustre 1.8.7
• Labels: llite performance
• Environment: Tested on RHEL 5.6, OFED 1.5.3.1, and Lustre 1.8.6
• Assignee: Andreas Dilger
• Reporter: Jeremy Filizetti

Description

Wile looking at functions with high CPU utilization for LU-1056 we came across lov_stripe_size as one of the top functions for files with larger striping. After looking into this it appears the reason can be summarized as:

With the current read implementation in Lustre 1.8 ll_readpage->ll_readahead->odd_merge_lvb is called for every page of a file at least once. Even if the read ahead pre-fetched the data into cache it still requires a subsequent call to ll_readpage to set the page flags to uptodate. obd_merge_lvb is used to calculate the kms for an inode and calls lov_stripe_size for every stripe in a file. So the larger the striping of a file the larger the overhead in this calculation. To reduce the overhead on calculating the kms i_size_read() could be used for the inode instead of locking the stripe and calling obd_merge_lvb. The inode size should be updated by ll_extent_lock which is called by ll_file_aio_read.

Issue Links

• Trackbacks: Lustre 1.8.x known issues tracker While testing against Lustre b18 branch, we would hit known bugs which were already reported in Lustre Bugzilla https://bugzilla.lustre.org/. In order to move away from relying on Bugzilla, we would create a JIRA

Comments

[John Fuchs-Chesney (Inactive)]

Issue may be raised again for 2.x at a future time.

[John Fuchs-Chesney (Inactive)]

Thank you Jeremy.
~ jfc.

[Jeremy Filizetti]

Go ahead and close the issue, I'll open a new ticket for Lustre 2.x when we get to that point.

[John Fuchs-Chesney (Inactive)]

Jeremy – is this still an issue you are interested in?
If you have moved on and we don't need to keep tracking this, I'd like to mark it as resolved.
Please let me know what you would prefer?
Thanks,
~ jfc.

[Andreas Dilger]

Oleg, you reworked a lot of the 1.8 page handling code to reduce the DLM overhead. Can you please comment on whether Jeremy's patch in http://review.whamcloud.com/2221 would work correctly?

Jeremy, is the performance in the attached graph with or without data checksums enabled? In 2.2 clients the checksums are calculated on multiple cores (even for a single-threaded read/write) and this has shown to improve single-client performance significantly.

[Peter Jones]

Andreas

Could you please answer Jeremy's follow-on question about this patch.

Thanks

Peter

[Jeremy Filizetti]

Thanks for the input Peter, I replied to Andreas's comments. I don't know if the same issue exists in master but we are still probably at least a year away from moving to Lustre 2+. Until then I still need to keep focusing on 1.8.

[Peter Jones]

Jeremy

Thanks for the patch. I know that Andreas has provided some comments on the patch in gerrit. My additional comment is that this work would be far likelier to land if it was targeted on master as that is the codeline under active development atm.

Peter

[Jeremy Filizetti]

Patch can be found at http://review.whamcloud.com/#change,2221

[Jeremy Filizetti]

Performance before and after patch reading uncached files from a client. While there is still a lot of fluctuations with the patch I was able to to obtain almost the same performance with 118 stripes as single striped file.