Re: Reviewing freeze map code
От | Masahiko Sawada |
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Тема | Re: Reviewing freeze map code |
Дата | |
Msg-id | CAD21AoBgUSo4KBzb-zotnWPJXPyaqQE4v8jfY4cwKkcaw9h52w@mail.gmail.com обсуждение исходный текст |
Ответ на | Re: Reviewing freeze map code (Robert Haas <robertmhaas@gmail.com>) |
Ответы |
Re: Reviewing freeze map code
|
Список | pgsql-hackers |
On Sat, May 7, 2016 at 5:34 AM, Robert Haas <robertmhaas@gmail.com> wrote: > On Mon, May 2, 2016 at 8:25 PM, Andres Freund <andres@anarazel.de> wrote: >> + * heap_tuple_needs_eventual_freeze >> + * >> + * Check to see whether any of the XID fields of a tuple (xmin, xmax, xvac) >> + * will eventually require freezing. Similar to heap_tuple_needs_freeze, >> + * but there's no cutoff, since we're trying to figure out whether freezing >> + * will ever be needed, not whether it's needed now. >> + */ >> +bool >> +heap_tuple_needs_eventual_freeze(HeapTupleHeader tuple) >> >> Wouldn't redefining this to heap_tuple_is_frozen() and then inverting the >> checks be easier to understand? > > I thought it much safer to keep this as close to a copy of > heap_tuple_needs_freeze() as possible. Copying a function and > inverting all of the return values is much more likely to introduce > bugs, IME. I agree. >> + /* >> + * If xmax is a valid xact or multixact, this tuple is also not frozen. >> + */ >> + if (tuple->t_infomask & HEAP_XMAX_IS_MULTI) >> + { >> + MultiXactId multi; >> + >> + multi = HeapTupleHeaderGetRawXmax(tuple); >> + if (MultiXactIdIsValid(multi)) >> + return true; >> + } >> >> Hm. What's the test inside the if() for? There shouldn't be any case >> where xmax is invalid if HEAP_XMAX_IS_MULTI is set. Now there's a >> check like that outside of this commit, but it seems strange to me >> (Alvaro, perhaps you could comment on this?). > > Here again I was copying existing code, with appropriate simplifications. > >> + * >> + * Clearing both visibility map bits is not separately WAL-logged. The callers >> * must make sure that whenever a bit is cleared, the bit is cleared on WAL >> * replay of the updating operation as well. >> >> I think including "both" here makes things less clear, because it >> differentiates clearing one bit from clearing both. There's no practical >> differentce atm, but still. > > I agree. Fixed. >> * >> * VACUUM will normally skip pages for which the visibility map bit is set; >> * such pages can't contain any dead tuples and therefore don't need vacuuming. >> - * The visibility map is not used for anti-wraparound vacuums, because >> - * an anti-wraparound vacuum needs to freeze tuples and observe the latest xid >> - * present in the table, even on pages that don't have any dead tuples. >> * >> >> I think the remaining sentence isn't entirely accurate, there's now more >> than one bit, and they're different with regard to scan_all/!scan_all >> vacuums (or will be - maybe this updated further in a later commit? But >> if so, that sentence shouldn't yet be removed...). > > We can adjust the language, but I don't really see a big problem here. This comment is not incorporate this patch so far. >> -/* Number of heap blocks we can represent in one byte. */ >> -#define HEAPBLOCKS_PER_BYTE 8 >> - >> Hm, why was this moved to the header? Sounds like something the outside >> shouldn't care about. > > Oh... yeah. Let's undo that. Fixed. >> #define HEAPBLK_TO_MAPBIT(x) (((x) % HEAPBLOCKS_PER_BYTE) * BITS_PER_HEAPBLOCK) >> >> Hm. This isn't really a mapping to an individual bit anymore - but I >> don't really have a better name in mind. Maybe TO_OFFSET? > > Well, it sorta is... but we could change it, I suppose. > >> +static const uint8 number_of_ones_for_visible[256] = { >> ... >> +}; >> +static const uint8 number_of_ones_for_frozen[256] = { >> ... >> }; >> >> Did somebody verify the new contents are correct? > > I admit that I didn't. It seemed like an unlikely place for a goof, > but I guess we should verify. >> /* >> - * visibilitymap_clear - clear a bit in visibility map >> + * visibilitymap_clear - clear all bits in visibility map >> * >> >> This seems rather easy to misunderstand, as this really only clears all >> the bits for one page, not actually all the bits. > > We could change "in" to "for one page in the". Fixed. >> * the bit for heapBlk, or InvalidBuffer. The caller is responsible for >> - * releasing *buf after it's done testing and setting bits. >> + * releasing *buf after it's done testing and setting bits, and must pass flags >> + * for which it needs to check the value in visibility map. >> * >> * NOTE: This function is typically called without a lock on the heap page, >> * so somebody else could change the bit just after we look at it. In fact, >> @@ -327,17 +351,16 @@ visibilitymap_set(Relation rel, BlockNumber heapBlk, Buffer heapBuf, >> >> I'm not seing what flags the above comment change is referring to? > > Ugh. I think that's leftover cruft from an earlier patch version that > should have been excised from what got committed. Fixed. >> /* >> - * A single-bit read is atomic. There could be memory-ordering effects >> + * A single byte read is atomic. There could be memory-ordering effects >> * here, but for performance reasons we make it the caller's job to worry >> * about that. >> */ >> - result = (map[mapByte] & (1 << mapBit)) ? true : false; >> - >> - return result; >> + return ((map[mapByte] >> mapBit) & VISIBILITYMAP_VALID_BITS); >> } >> >> Not a new issue, and *very* likely to be irrelevant in practice (given >> the value is only referenced once): But there's really no guarantee >> map[mapByte] is only read once here. > > Meh. But we can fix if you want to. Fixed. >> -BlockNumber >> -visibilitymap_count(Relation rel) >> +void >> +visibilitymap_count(Relation rel, BlockNumber *all_visible, BlockNumber *all_frozen) >> >> Not really a new issue again: The parameter types (previously return >> type) to this function seem wrong to me. > > Not this patch's job to tinker. This comment is not incorporate this patch yet. >> @@ -1934,5 +1992,14 @@ heap_page_is_all_visible(Relation rel, Buffer buf, TransactionId *visibility_cut >> } >> + /* >> + * We don't bother clearing *all_frozen when the page is discovered not >> + * to be all-visible, so do that now if necessary. The page might fail >> + * to be all-frozen for other reasons anyway, but if it's not all-visible, >> + * then it definitely isn't all-frozen. >> + */ >> + if (!all_visible) >> + *all_frozen = false; >> + >> >> Why don't we just set *all_frozen to false when appropriate? It'd be >> just as many lines and probably easier to understand? > > I thought that looked really easy to mess up, either now or down the > road. This way seemed more solid to me. That's a judgement call, of > course. To be understanding easier, I changed it so. >> + /* >> + * If the page is marked as all-visible but not all-frozen, we should >> + * so mark it. Note that all_frozen is only valid if all_visible is >> + * true, so we must check both. >> + */ >> >> This kinda seems to imply that all-visible implies all_frozen. Also, why >> has that block been added to the end of the if/else if chain? Seems like >> it belongs below the (all_visible && !all_visible_according_to_vm) block. > > We can adjust the comment a bit to make it more clear, if you like, > but I doubt it's going to cause serious misunderstanding. As for the > placement, the reason I put it at the end is because I figured that we > did not want to mark it all-frozen if any of the "oh crap, emit a > warning" cases applied. > Fixed comment. I think that we should care about all-visible problem first, and then care all-frozen problem. So this patch doesn't change the placement. Attached patch fixes only above comments, other are being addressed now. -- Regards, -- Masahiko Sawada
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