Re: [PoC] Improve dead tuple storage for lazy vacuum - Mailing list pgsql-hackers
| From | John Naylor |
|---|---|
| Subject | Re: [PoC] Improve dead tuple storage for lazy vacuum |
| Date | |
| Msg-id | CAFBsxsF_y9Nwkk2-3pktAkPZq6NoRGRkGSFoto9isytQfNOUSA@mail.gmail.com Whole thread Raw |
| In response to | Re: [PoC] Improve dead tuple storage for lazy vacuum (Masahiko Sawada <sawada.mshk@gmail.com>) |
| Responses |
Re: [PoC] Improve dead tuple storage for lazy vacuum
|
| List | pgsql-hackers |
I wrote:
> > > Since the block-level measurement is likely overestimating quite a bit, I propose to simply reverse the order of the actions here, effectively reporting progress for the *last page* and not the current one: First update progress with the current memory usage, then add tids for this page. If this allocated a new block, only a small bit of that will be written to. If this block pushes it over the limit, we will detect that up at the top of the loop. It's kind of like our earlier attempts at a "fudge factor", but simpler and less brittle. And, as far as OS pages we have actually written to, I think it'll effectively respect the memory limit, at least in the local mem case. And the numbers will make sense.
> > >
> > > Thoughts?
> >
> > It looks to work but it still doesn't work in a case where a shared
> > tidstore is created with a 64kB memory limit, right?
> > TidStoreMemoryUsage() returns 1MB and TidStoreIsFull() returns true
> > from the beginning.
>
> I have two ideas:
>
> 1. Make it optional to track chunk memory space by a template parameter. It might be tiny compared to everything else that vacuum does. That would allow other users to avoid that overhead.
> 2. When context block usage exceeds the limit (rare), make the additional effort to get the precise usage -- I'm not sure such a top-down facility exists, and I'm not feeling well enough today to study this further.
Since then, Masahiko incorporated #1 into v31, and that's what I'm looking at now. Unfortunately, If I had spent five minutes reminding myself what the original objections were to this approach, I could have saved us some effort. Back in July (!), Andres raised two points: GetMemoryChunkSpace() is slow [1], and fragmentation [2] (leading to underestimation).
In v31, in the local case at least, the underestimation is actually worse than tracking chunk space, since it ignores chunk header and alignment. I'm not sure about the DSA case. This doesn't seem great.
> > > Since the block-level measurement is likely overestimating quite a bit, I propose to simply reverse the order of the actions here, effectively reporting progress for the *last page* and not the current one: First update progress with the current memory usage, then add tids for this page. If this allocated a new block, only a small bit of that will be written to. If this block pushes it over the limit, we will detect that up at the top of the loop. It's kind of like our earlier attempts at a "fudge factor", but simpler and less brittle. And, as far as OS pages we have actually written to, I think it'll effectively respect the memory limit, at least in the local mem case. And the numbers will make sense.
> > >
> > > Thoughts?
> >
> > It looks to work but it still doesn't work in a case where a shared
> > tidstore is created with a 64kB memory limit, right?
> > TidStoreMemoryUsage() returns 1MB and TidStoreIsFull() returns true
> > from the beginning.
>
> I have two ideas:
>
> 1. Make it optional to track chunk memory space by a template parameter. It might be tiny compared to everything else that vacuum does. That would allow other users to avoid that overhead.
> 2. When context block usage exceeds the limit (rare), make the additional effort to get the precise usage -- I'm not sure such a top-down facility exists, and I'm not feeling well enough today to study this further.
Since then, Masahiko incorporated #1 into v31, and that's what I'm looking at now. Unfortunately, If I had spent five minutes reminding myself what the original objections were to this approach, I could have saved us some effort. Back in July (!), Andres raised two points: GetMemoryChunkSpace() is slow [1], and fragmentation [2] (leading to underestimation).
In v31, in the local case at least, the underestimation is actually worse than tracking chunk space, since it ignores chunk header and alignment. I'm not sure about the DSA case. This doesn't seem great.
It shouldn't be a surprise why a simple increment of raw allocation size is comparable in speed -- GetMemoryChunkSpace() calls the right function through a pointer, which is slower. If we were willing to underestimate for the sake of speed, that takes away the reason for making memory tracking optional.
Further, if the option is not specified, in v31 there is no way to get the memory use at all, which seems odd. Surely the caller should be able to ask the context/area, if it wants to.
I still like my idea at the top of the page -- at least for vacuum and m_w_m. It's still not completely clear if it's right but I've got nothing better. It also ignores the work_mem issue, but I've given up anticipating all future cases at the moment.
I'll put this item and a couple other things together in a separate email tomorrow.
[1] https://www.postgresql.org/message-id/20220704211822.kfxtzpcdmslzm2dy%40awork3.anarazel.de
[2] https://www.postgresql.org/message-id/20220704220038.at2ane5xkymzzssb%40awork3.anarazel.de
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