[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 |
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| 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: |
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| Backwards Compatibility: | Fully Compatible | ||||||||
| Backport Requested: |
v4.2
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| Sprint: | Execution Team 2019-07-29, Execution Team 2019-08-12 | ||||||||
| Participants: | |||||||||
| Description |
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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 ] |
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Author: {'username': 'DiannaHohensee', 'email': 'dianna.hohensee@10gen.com', 'name': 'Dianna Hohensee'}Message: (cherry picked from commit ab48766f3ab9953bf0beb60a64bf8da3a2d0dd0b) |
| Comment by Githook User [ 01/Aug/19 ] |
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Author: {'name': 'Dianna Hohensee', 'email': 'dianna.hohensee@10gen.com', 'username': 'DiannaHohensee'}Message: |
| Comment by Dianna Hohensee (Inactive) [ 15/Jul/19 ] |
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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 ] |
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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 ] |
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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 ] |
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At what lookaside score does eviction begin in earnest? |