[SERVER-42205] Raise the snapshot window's cache pressure threshold from 50 to 95 Created: 12/Jul/19  Updated: 29/Oct/23  Resolved: 01/Aug/19

Status: Closed
Project: Core Server
Component/s: Storage
Affects Version/s: None
Fix Version/s: 4.2.1, 4.3.1

Type: Improvement Priority: Major - P3
Reporter: Dianna Hohensee (Inactive) Assignee: Dianna Hohensee (Inactive)
Resolution: Fixed Votes: 0
Labels: None
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Issue Links:
Backports
Related
Backwards Compatibility: Fully Compatible
Backport Requested:
v4.2
Sprint: Execution Team 2019-07-29, Execution Team 2019-08-12
Participants:

 Description   

WT doesn't trigger eviction until the LAS score reaches a high value. I think 95 is safer for the snapshot window cache pressure threshold, so that we do not stop servicing 'snapshot' read concern operations while the cache pressure remains unchanged because WT doesn't think it high enough for concern yet.



 Comments   
Comment by Githook User [ 15/Aug/19 ]

Author:

{'username': 'DiannaHohensee', 'email': 'dianna.hohensee@10gen.com', 'name': 'Dianna Hohensee'}

Message: SERVER-42205 Raise the snapshot window's cache pressure threshold from 50 to 95

(cherry picked from commit ab48766f3ab9953bf0beb60a64bf8da3a2d0dd0b)
Branch: v4.2
https://github.com/mongodb/mongo/commit/0659c76410c975d826db02e69119a6a1a1fccf98

Comment by Githook User [ 01/Aug/19 ]

Author:

{'name': 'Dianna Hohensee', 'email': 'dianna.hohensee@10gen.com', 'username': 'DiannaHohensee'}

Message: SERVER-42205 Raise the snapshot window's cache pressure threshold from 50 to 95
Branch: master
https://github.com/mongodb/mongo/commit/ab48766f3ab9953bf0beb60a64bf8da3a2d0dd0b

Comment by Dianna Hohensee (Inactive) [ 15/Jul/19 ]

Alex suggested 95 in an old email, I believe as part of choosing non-invasive defaults to start. It sounds reasonable to me. It seems pretty hard to finely guess at which LAS score threshold would or would not push us into disk usage if we kept another X (additive increase setting) seconds worth of history.

Comment by Eric Milkie [ 15/Jul/19 ]

So by choosing 95 this should be a good balance between avoiding cache pressure and continuing to service snapshot reads? Sounds okay by me then.

Comment by Dianna Hohensee (Inactive) [ 15/Jul/19 ]

Alex said "Eviction doesn't rely on the las score. Eviction that writes to cache overflow (aka lookaside) requires either a score of 100 or the cache to be operating in aggressive mode".

It seems that even at 95, cache pressure can persist. So whatever LAS score threshold we set, it's really the point at which we decide to stop allowing data history to push pressure even higher, to start causing disk overflow and slow cache.

Comment by Eric Milkie [ 15/Jul/19 ]

At what lookaside score does eviction begin in earnest?

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