On 2015-09-02 22:46:59 +0100, Greg Stark wrote:
> On Sun, Jul 26, 2015 at 1:46 PM, Andres Freund <andres@anarazel.de> wrote:
> > To me it sounds like this shouldn't go through the full ReadBuffer()
> > rigamarole. That code is already complex enough, and here it's really
> > not needed. I think it'll be much easier to review - and actually faster
> > in many cases to simply have something like
> >
> > bool
> > BufferInCache(Relation, ForkNumber, BlockNumber)
> > {
> > /* XXX: setup tag, hash, partition */
> >
> > LWLockAcquire(newPartitionLock, LW_SHARED);
> > buf_id = BufTableLookup(&newTag, newHash);
> > LWLockRelease(newPartitionLock);
> > return buf_id != -1;
> > }
> >
> > and then fall back to the normal ReadBuffer() when it's in cache.
>
>
> I guess I'm missing something but how is this API useful? As soon as
> its returned it the result might be invalid since before you actually
> make use of the return value another process could come and read in
> and pin the page in question. Is this part of some interlock where you
> only have to know it was unpinned at some point in time since some
> other event?
Yes. We're talking about this block: for (blkno = xlrec->lastBlockVacuumed + 1; blkno < thisblkno; blkno++) {
/* * We use RBM_NORMAL_NO_LOG mode because it's not an error * condition to see all-zero pages. The
originalbtvacuumpage * scan would have skipped over all-zero pages, noting them in FSM * but not
botheringto initialize them just yet; so we mustn't * throw an error here. (We could skip acquiring the
cleanuplock * if PageIsNew, but it's probably not worth the cycles to test.) * * XXX we don't
actuallyneed to read the block, we just need to * confirm it is unpinned. If we had a special call into the
* buffer manager we could optimise this so that if the block is * not in shared_buffers we confirm it as
unpinned. */ buffer = XLogReadBufferExtended(thisrnode, MAIN_FORKNUM, blkno,
RBM_NORMAL_NO_LOG); if (BufferIsValid(buffer)) { LockBufferForCleanup(buffer);
UnlockReleaseBuffer(buffer); } }}
Greetings,
Andres Freund
