On Tue, Sep 27, 2022 at 4:50 PM Tom Lane <t...@sss.pgh.pa.us> wrote: > I wrote: > > * frame #0: 0x000000010a36af8c postgres`ParseCommitRecord(info='\x80', > > xlrec=0x00007fa0678a8090, parsed=0x00007ff7b5c50e78) at xactdesc.c:102:30 > > Okay, so the problem is this: by widening RelFileNumber to 64 bits, > you have increased the alignment requirement of struct RelFileLocator, > and thereby also SharedInvalidationMessage, to 8 bytes where it had > been 4. longfin's alignment check is therefore expecting that > xl_xact_twophase will likewise be 8-byte-aligned, but it isn't:
Yeah, I reached the same conclusion. > There is a second problem that I am going to hold your feet to the > fire about: > > (lldb) p sizeof(SharedInvalidationMessage) > (unsigned long) $1 = 24 > > We have sweated a good deal for a long time to keep that struct > to 16 bytes. I do not think 50% bloat is acceptable. I noticed that problem, too. The attached patch, which perhaps you can try out, fixes the alignment issue and also reduces the size of SharedInvalidationMessage from 24 bytes back to 20 bytes. I do not really see a way to do better than that. We use 1 type byte, 3 bytes for the backend ID, 4 bytes for the database OID, and 4 bytes for the tablespace OID. Previously, we then used 4 bytes for the relfilenode, but now we need 7, and there's no place from which we can plausibly steal those bits, at least not as far as I can see. If you have ideas, I'm all ears. Also, I don't really know what problem you think it's going to cause if that structure gets bigger. If we increased the size from 16 bytes even all the way to 32 or 64 bytes, what negative impact do you think that would have? It would use a little bit more shared memory, but on modern systems I doubt it would be enough to get excited about. The bigger impact would probably be that it would make commit records a bit bigger since those carry invalidations as payload. That is not great, but I think it only affects commit records for transactions that do DDL, so I'm struggling to see that as a big performance problem. -- Robert Haas EDB: http://www.enterprisedb.com
v1-0001-tweak-SharedInvalSmgrMsg.patch
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