I recently updated to 24.06.02, re-optimized and rescheduled – as I’ve done several times over the past 8 months using FSRS – and I am surprised to see an unexpectedly huge backlog. I am unable to answer for myself the sorts of questions I frequently answer for others. 
Is this within the realm of normal? Am I doing something wrong?
TL;DR:
- Why am I getting a huge reschedule-backlog? 400 cards after a normal re-optimize, and then 900 cards instead after updating versions (and a re-optimize) a week later?
- Is FSRS in 24.06.02 so different from 23.12.1 that it accounts for this? What changed that would cause a 10% jump in my review history?
- What does it mean that FSRS tells me it’s doing a great job fitting my review history – and has told me throughout the time I’ve been using it – if I’m getting a sudden backlog like this? Am I really so unpredictable?
- Should I expect to keep getting a huge backlog every time I update?
My optimization “habit” –
Since I started using FSRS, every month or 2 – whenever I remember, or if I make a significant change (like changing what decks are in a preset) – I evaluate my current parameters, re-optimize, and re-evaluate. A little before-and-after check for how things are going.
Details
There has only been one time when my RMSE got worse after re-optimization. (I didn’t switch to the new parameters then, and I know that has been fixed to not be an issue again.) After getting new parameters, I usually reschedule, but I’m not a fanatic about it.
When I initially started using custom-code FSRS last year, I rescheduled-all using the Helper add-on (this was back before the recommendation against doing that!) – and I was rewarded with a backlog of about 400 cards. It was miserable for 2 months. Since then, my typical reschedule-backlog might be about 30 cards.
On a completely unrelated cycle, I periodically reschedule-all with the add-on to get load balancing. This is probably once a month or so, or after I have had some especially uneven days (incomplete Reviews, backlog catch-up, etc.). While I keep a careful record of my optimizations / parameters / evaluates (shocking, I’m sure) – I don’t really make a note of my reschedule-alls.
My last “typical” re-optimize:
4-15-24 using 23.12.1
evaluated 56K reviews at RMSE 1.72%
re-optimized, evaluated at RMSE 1.57%
Parameters
0.3050, 1.4696, 5.2671, 18.7406, 5.3726, 1.4943, 0.8948, 0.0241, 2.0988, 0.2850, 1.4503, 2.3259, 0.0354, 0.2415, 1.8678, 0.1807, 2.8789
I likely did a reschedule-all then; I can’t be sure. However, I know for sure that on 6-5-24, I did a reschedule-all to demonstrate Load Balance for someone, and there wasn’t much change. I can’t remember if I left that in place or undid it.
My next re-optimize:
6-24-24 using 23.12.1
evaluated 60K reviews at RMSE 1.49%
re-optimized, evaluated at RMSE 1.28%
Parameters
0.3212, 1.4901, 5.4759, 20.6647, 5.5531, 1.3864, 0.9140, 0.0200, 1.9740, 0.2504, 1.3648, 2.3066, 0.0528, 0.2716, 1.8885, 0.0842, 2.7771
Odd thing #1
After that, I ran a reschedule-all with the add-on, and it surprised me with a backlog of over 400 cards. That’s big for my collection (6% of my active cards), and more than a week’s workload. This was unexpected to me after using FSRS for 8+ months (the biggest reschedule-backlog since I first enabled FSRS), and with nothing significant changing in the past 2 months. I undid that reschedule and thought about what to do next.
The current situation:
A few days later on 6-27-24, I upgraded to 24.06.02. And then today, I took another run at re-optimizing.
7-1-24 using 24.06.02
evaluated 67K reviews at RMSE 2.28%
re-optimized, evaluated at 2.05%
Parameters
0.2712, 1.2870, 4.5993, 36.3857, 5.5666, 1.8591, 1.0206, 0.0141, 2.1699, 0.3620, 1.6549, 2.3402, 0.0826, 0.2489, 2.6217, 0.1198, 2.7451
Odd thing #2
Evaluated 67K reviews?! There’s no way I did ~7000 reviews in a week! I checked and I only studied 386 cards – FSRS Stats is counting 390 reps, main Stats is counting 600 reps. There were no major changes to my collection, the same 6 decks are in this preset, and only 3 of those have cards in them. I edited 100-ish notes, added a few others, and likely suspended / unsuspended some cards.
Odd thing #3
RMSE went the wrong way, to 2.28%. I guess a new version of FSRS can come with significant changes. But I optimized just a week ago, so that seems like a lot of change.
After the re-optimize, my new parameters shifted more-than-usual (by visual approximation) – not entirely expected with a different FSRS version, it seems like a lot after such a short time. The RMSE improved a little, but not as good as I expected from before updating.
Odd thing #4
Ran reschedule-all, and got a backlog of over 900 cards! What?! How is it possible that almost 14% of my active cards needed to be rescheduled?
Some notes on Stats
My desired retention has been 92% throughout – steady with what I was maintaining using SM-2. True retention has dropped a bit below that the past months or so, to ~91%.
Taking a look at my other Stats, with the current re-optimize, my average stability (for 95%) dropped from 1.49y to 1.28y. Then with the reschedule, my average interval (for 95%) had a commensurate drop from 1.26y to 1.03y.