Thread: Free-space-map management thoughts
I've been thinking about improving the algorithm that the free space map (FSM) uses to decide what to store when it's not got enough shared memory to keep track of everything. The present design uses a dynamically adjusted threshold for each relation, throwing away pages whose free space amount is less than the threshold. This unfortunately seems not to work as well as I'd hoped when I wrote it :-(. In particular it cannot cope effectively with situations where many pages have exactly the same amount of free space --- it degenerates to all-or-nothing. This problem has become acute now that btree indexes use the FSM to keep track of free pages: by definition, all those pages have exactly the same amount of free space. I had some off-list discussions last fall with Steve Marshall, who was trying to improve the thresholding algorithm to work better, but what he ended up with seemed to me to have a lot of ad-hoc logic and magic numbers in it. So I wasn't real satisfied with that. This is a request for comments about the following redesign: 1. Continue to keep track of the average request size seen by GetPageWithFreeSpace(), but make it a pure request-size average; don't muck it up with thresholding adjustments. Add an entry point to make the average request size for a relation available to callers. VACUUM can use this as a starting point for its internal decisions about which pages are even worth reporting to the FSM. 2. Eliminate "retail" addition of FSM entries. RecordFreeSpace() isn't being used anyway, and we can restrict RecordAndGetPageWithFreeSpace() to only update existing entries not make new ones. Only wholesale replacement of a relation's FSM data, via MultiRecordFreeSpace(), need be supported as a way of adding page entries to FSM. 3. With the above two changes, the numbers of pages passed to the FSM by MultiRecordFreeSpace() calls become useful statistics in themselves. We can keep track of those numbers in the FSM's per-relation statistics independently of the number of pages actually stored in FSM. 4. In particular, the sum of the MultiRecordFreeSpace (henceforth MRFS) page counts gives us the total number of pages we would *like* to keep track of; the ratio of this number to the actual allocated max_fsm_pages is our "oversubscription ratio". (One thing we can do in passing is make VACUUM VERBOSE print these numbers, so that people finally have some intelligent way of adjusting their FSM parameters.) 5. When FSM space is oversubscribed, we can divide each relation's MRFS requested page count by the oversubscription ratio to arrive at an exact target page count for each relation. This page count is stable as long as the space-allocation picture isn't changing much, which is a big improvement over the existing inherently history-dependent thresholding algorithm. 6. A reasonably effective way to reduce a relation's stored page list to the target (or select out the pages to actually remember from an MRFS request) is as follows: * Prescan the page data to compute a histogram of available-space values, with maybe 32 bins. *Find the histogram bin whose inclusion would make us exceed the target page count. Set thresholdL = its lower edge value,thresholdU = its upper edge value, and bincount = target page count minus sum of counts in higher histogram bins.* Scan pages. Keep all those >= thresholdU; drop those < thresholdL; of the pages between the thresholds, keep thefirst bincount entries. This approach will drop some pages with more free space while keeping some with less --- but thedifference between those dropped and those kept is no more than the bin width. This algorithm still requires some heuristics around the edges --- in particular, in step 5 we probably don't want a purely linear division of available space, but should allow some minimum number of pages for each relation before divvying up the leftover space. But it doesn't seem to need nearly as much ad-hoc tuning as the thresholding approach does. One thing this does not do that the original code tries to do is give preference to more-heavily-used relations --- in this approach, each table's share of FSM is determined only by its number of pages with free space. However, one could argue that that is an indirect measure of activity, since it certainly reflects past deletion/update activity. So it may be okay not to give any explicit preference to active tables. Comments anyone? regards, tom lane PS: Another idea I'm toying with is to dump out the FSM contents at postmaster shutdown and reload them at restart, so that the FSM doesn't have to start from ground zero on every restart cycle. But that's independent of the management algorithm...
> PS: Another idea I'm toying with is to dump out the FSM contents at > postmaster shutdown and reload them at restart, so that the FSM doesn't > have to start from ground zero on every restart cycle. But that's > independent of the management algorithm... Correct me if I'm wrong, but the FSM is only populated by vacuum, so there is no FSM information for any given table / database until it's vacuumed, in a long running production enviornment this may not be that important, but it could result in a large increase in file size any time the database is restarted. I think this change, while independent of your proposal, is important.
"Matthew T. O'Connor" <matthew@zeut.net> writes: > Correct me if I'm wrong, but the FSM is only populated by vacuum, so there > is no FSM information for any given table / database until it's vacuumed, in > a long running production enviornment this may not be that important, but it > could result in a large increase in file size any time the database is > restarted. Right. The original assumption was that people don't restart production servers, so startup-transient behavior isn't very important. But that's obviously not a great assumption. It seemed expedient at the time (partly because I wasn't sure FSM would fly at all) --- but now it's time to go back and fill in the holes. regards, tom lane
Stephen Marshall <smarshall@wsi.com> writes: > 1. When the FSM is oversubscribed and one is trying to decide which > pages to keep, remember that page info is stored in groups of > CHUNK_SIZE pages, where CHUNK_SIZE is current 32. Right, oversubscription would actually need to be measured in chunks not single pages. > 2. The histogram concept is a neat idea, but I think some reorganization > of the page information might make it unnecessary. Currently the FSM > pages are sorted by BlockNumber. This was particularly useful for > adding information about a single page, but since that interface is no > longer to be supported, perhaps the decision to sort by BlockNumber > should also be revisited. I was thinking about that, but we do still need to handle RecordAndGetFreeSpace --- in fact that should be the most common operation. The histogram approximation seems an okay price to pay for not slowing down RecordAndGetFreeSpace. If you wanted to depend on the ordering-by-free-space property to any large extent, RecordAndGetFreeSpace would actually have to move the old page down in the list after adjusting its free space :-( > If we sort the page info by available space, we could then use binary > search to find space thresholds when we are handling oversubscription. The list-of-chunks storage layout provides only limited traction for searching anyway, and none at all for a binary search AFAICS. I toyed with smarter data structures such as hashes or btrees, but couldn't convince myself that the extra space would be justified. > Am I missing something that requires the FSM to be ordered by block number? We could doubtless make it work either way, but I think optimizing VACUUM at the expense of RecordAndGetFreeSpace is probably not the way to go. Another factor to consider is that the round-robin algorithm for handing out pages during GetFreeSpace would behave considerably differently if the block list is sorted by free space not block number. I'm not sure offhand that it would be worse, but we'd have to think about the consequences. regards, tom lane
On Wed, 2003-02-26 at 15:54, Tom Lane wrote: <snip> > > Comments anyone? > Now that indexes are getting some reporting, my understanding is an index would report fewer pages overall than it's associated table, but those pages would be completely empty. However, given that they don't reported non-empty pages, the percentage of freeable space to total space would be unfairly lower (if I'm right in thinking that the back end will assume that non-reported pages don't have empty space in them). This would tend to hurt index management even though it's pages are the best candidates for removal (100% empty). Is this a valid concern, or am I misreading something? Robert Treat
Robert Treat <xzilla@users.sourceforge.net> writes: > Now that indexes are getting some reporting, my understanding is an > index would report fewer pages overall than it's associated table, but > those pages would be completely empty. However, given that they don't > reported non-empty pages, the percentage of freeable space to total > space would be unfairly lower (if I'm right in thinking that the back > end will assume that non-reported pages don't have empty space in them). > This would tend to hurt index management even though it's pages are the > best candidates for removal (100% empty). Is this a valid concern, or am > I misreading something? I'm not following your point... across relations, the proposed scheme only considers numbers of pages, not how much space is believed free in each such page. If anything I suspect it would over-favor the indexes. regards, tom lane
On Thu, 2003-02-27 at 11:00, Tom Lane wrote: > Robert Treat <xzilla@users.sourceforge.net> writes: > > Now that indexes are getting some reporting, my understanding is an > > index would report fewer pages overall than it's associated table, but > > those pages would be completely empty. However, given that they don't > > reported non-empty pages, the percentage of freeable space to total > > space would be unfairly lower (if I'm right in thinking that the back > > end will assume that non-reported pages don't have empty space in them). > > This would tend to hurt index management even though it's pages are the > > best candidates for removal (100% empty). Is this a valid concern, or am > > I misreading something? > > I'm not following your point... across relations, the proposed scheme > only considers numbers of pages, not how much space is believed free in > each such page. If anything I suspect it would over-favor the indexes. > I think I was thinking that a given table will always report more pages than an index on that table, since tables can report 50% empty pages while indexes only report 100% empty pages. This would cause tables to generally be favored over indexes, even though the index pages have the most to gain. Robert Treat
Robert Treat <xzilla@users.sourceforge.net> writes: > I think I was thinking that a given table will always report more pages > than an index on that table, since tables can report 50% empty pages > while indexes only report 100% empty pages. This would cause tables to > generally be favored over indexes, even though the index pages have the > most to gain. But the other side of that coin is that the table needs the FSM service more: it will be accepting larger insertions and trying to stuff them into pages that aren't wholly empty. So it just naturally needs to keep track of more free pages. (This argument is rigorously true if you compare a table with its own index; it's probably qualitatively okay when considering unrelated indexes.) So like I said, I'm really more concerned that the indexes may be over-favored. Also keep in mind that, as Steve pointed out, we'll really be allocating space on a chunk basis not a page basis. For an index there is no need to store free-space-per-page at all; this means we could fit 48 page numbers into the same size chunk that normally holds 32 page numbers and 32 free space counts. (I'm not sure I will do this, but I will if it can be done without uglifying the code excessively.) So if we allocate space on a proportionate-number-of-chunks basis, the indexes should get some additional win there. We could throw in a fudge-factor multiplier that discriminates for or against indexes while choosing how much space to give them. Without any clear idea where to set it, I'm not eager to do so --- but that'd provide a solution if it becomes apparent that one side or the other is being favored too much. regards, tom lane
I wrote: > Stephen Marshall <smarshall@wsi.com> writes: >> If we sort the page info by available space, we could then use binary >> search to find space thresholds when we are handling oversubscription. > The list-of-chunks storage layout provides only limited traction for > searching anyway, and none at all for a binary search AFAICS. I toyed > with smarter data structures such as hashes or btrees, but couldn't > convince myself that the extra space would be justified. Here's a possibly off-the-wall idea: maybe the list-of-chunks representation is not too simple, but too complex. Suppose we stored all the FSM page data as one big array (of ItemPointerData elements). Any given relation would own a section of this array in which its page data is sorted in page-number order. Then RecordAndGetFreeSpace could use a binary search to locate the old page it needs to update. This would be noticeably faster as far as lookup operations go, but the Achilles' heel would be inserting new data: in general you'd need to push stuff around in the page array to make room. Given fast memcpy() this might not be too bad though. Alternatively, the data-copying could be combined with the scan we'd likely be making to remove undersized pages. Anyone like this idea? Or should I leave well enough alone? regards, tom lane
Stephen Marshall <smarshall@wsi.com> writes: > If I understand the concept correctly, the histogram will only be > calculated when MultiRecordFreeSpace is called AND the FSM is > oversubscribed. However, when it is called, we will need to calculate a > histogram for, and potentially trim data from, all relations that have > entries in the FSM. Right --- at least for calls where the particular relation's request has gone up from before. We could skip examining the other rels when the requested number of pages is the same or less. > When vacuuming the entire database, we will end up with an N-squared > loop where we iterate over all the relations in vacuum, and iterate over > them again in each call to MultiRecordFreeSpace that occurs within each > vacuum. Hmm ... good point. I was thinking it was linear, but that's per MultiRecordFreeSpace call. The total work involved for a whole-database vacuum does look like O(N^2). Is there a way around that? It may be that this isn't really a problem; the behavior of the existing code could be characterized the same way. (In fact I think it could be worse than O(N^2) depending on how often acquire_free_space ends up getting called.) But it's something to think about. > In any event, I don't really think this is a problem, just something to > pay attention to. It also highlights the need to make the histogram > calculation and free space adjustment as efficient as possible. Yeah. We need that anyway since we'll be doing it with the FSM lock held. (I was trying to think of ways to avoid holding the lock throughout MultiRecordFreeSpace, but came up dry.) regards, tom lane
Tom Lane wrote:<br /><blockquote cite="mid10845.1046357945@sss.pgh.pa.us" type="cite"><pre wrap="">Stephen Marshall <a class="moz-txt-link-rfc2396E"href="mailto:smarshall@wsi.com"><smarshall@wsi.com></a> writes: </pre><blockquote type="cite"><prewrap="">2. The histogram concept is a neat idea, but I think some reorganization of the page information might make it unnecessary. Currently the FSM pages are sorted by BlockNumber. This was particularly useful for adding information about a single page, but since that interface is no longer to be supported, perhaps the decision to sort by BlockNumber should also be revisited. </pre></blockquote><pre wrap=""> I was thinking about that, but we do still need to handle RecordAndGetFreeSpace --- in fact that should be the most common operation. The histogram approximation seems an okay price to pay for not slowing down RecordAndGetFreeSpace. If you wanted to depend on the ordering-by-free-space property to any large extent, RecordAndGetFreeSpace would actually have to move the old page down in the list after adjusting its free space :-( </pre></blockquote> I hadn't considered the needs of RecordAndGetFreeSpace. Itis called so much more than MultiRecordFreeSpace that it make much better sense to optimize it, and hence organize thepage information by BlockNumber.<br /><br /> I think you just sold me on the histogram idea :) but I still have somethoughts about its behavior in the oversubscribed state.<br /><br /> If I understand the concept correctly, the histogramwill only be calculated when MultiRecordFreeSpace is called AND the FSM is oversubscribed. However, when it iscalled, we will need to calculate a histogram for, and potentially trim data from, all relations that have entries in theFSM. <br /><br /> When vacuuming the entire database, we will end up with an N-squared loop where we iterate over allthe relations in vacuum, and iterate over them again in each call to MultiRecordFreeSpace that occurs within each vacuum. If each relation consistantly requests the storage of the same amount of page info during each vacuum, the extrawork of this N-squared loop will probably disappear after the system settles into an equilibrium, but inconsistant requestscould cause more oscillations in the free space adjustment.<br /><br /> Do I understand how this will work properly,or did I miss something?<br /><br /> In any event, I don't really think this is a problem, just something to payattention to. It also highlights the need to make the histogram calculation and free space adjustment as efficient aspossible.<br /><br /> By-the-way, I think your other suggestions are great (e.g. changes to the public API, maintainingmore internal statics, reporting more info in VACUUM VERBOSE, ensuring that a minimum amout of freespace infois retained for all relations). I think this will be a nice improvement to how postgres reclaims disk space.<br /><br/>
Tom, I'm happy to see your attentions turning back to the FSM. I like the design, but I do have a few suggestions, particularly about how to handle oversubscription of the FSM. 1. When the FSM is oversubscribed and one is trying to decide which pages to keep, remember that page info is stored in groups of CHUNK_SIZE pages, where CHUNK_SIZE is current 32. Thus, if you need to store info for 1 page, you have already committed space in the FSM for CHUNK_SIZE pages, so you might as well fill that chunk with valid page information. Such logic is not needed just for optimization, but also to prevent the oversubscription logic from trying to use more chunks than the FSM has. 2. The histogram concept is a neat idea, but I think some reorganization of the page information might make it unnecessary. Currently the FSM pages are sorted by BlockNumber. This was particularly useful for adding information about a single page, but since that interface is no longer to be supported, perhaps the decision to sort by BlockNumber should also be revisited. If we sort the page info by available space, we could then use binary search to find space thresholds when we are handling oversubscription. I think this would be both faster and more exact thanthe histogram approach. Sorting by available space would make the sgmr code a bit less efficient, as we would not be able to use binary search to skip to the min block number provided in MultiRecordFreeSpace. However, lazy vacuuming would be more efficient, as this function starts with pages ordered by available space, then sorts the page info by block number just prior to the call of MultiRecordFreeSpace. Am I missing something that requires the FSM to be ordered by block number? Yours, Steve Marshall ----- Tom Lane wrote: >I've been thinking about improving the algorithm that the free space map >(FSM) uses to decide what to store when it's not got enough shared >memory to keep track of everything. The present design uses a dynamically >adjusted threshold for each relation, throwing away pages whose free >space amount is less than the threshold. This unfortunately seems not >to work as well as I'd hoped when I wrote it :-(. In particular it >cannot cope effectively with situations where many pages have exactly >the same amount of free space --- it degenerates to all-or-nothing. >This problem has become acute now that btree indexes use the FSM to >keep track of free pages: by definition, all those pages have exactly >the same amount of free space. > >I had some off-list discussions last fall with Steve Marshall, who >was trying to improve the thresholding algorithm to work better, but >what he ended up with seemed to me to have a lot of ad-hoc logic and >magic numbers in it. So I wasn't real satisfied with that. > >This is a request for comments about the following redesign: > >1. Continue to keep track of the average request size seen by >GetPageWithFreeSpace(), but make it a pure request-size average; don't >muck it up with thresholding adjustments. Add an entry point to make >the average request size for a relation available to callers. VACUUM >can use this as a starting point for its internal decisions about which >pages are even worth reporting to the FSM. > >2. Eliminate "retail" addition of FSM entries. RecordFreeSpace() isn't >being used anyway, and we can restrict RecordAndGetPageWithFreeSpace() >to only update existing entries not make new ones. Only wholesale >replacement of a relation's FSM data, via MultiRecordFreeSpace(), need >be supported as a way of adding page entries to FSM. > >3. With the above two changes, the numbers of pages passed to the FSM >by MultiRecordFreeSpace() calls become useful statistics in themselves. >We can keep track of those numbers in the FSM's per-relation statistics >independently of the number of pages actually stored in FSM. > >4. In particular, the sum of the MultiRecordFreeSpace (henceforth MRFS) >page counts gives us the total number of pages we would *like* to keep >track of; the ratio of this number to the actual allocated max_fsm_pages >is our "oversubscription ratio". (One thing we can do in passing is >make VACUUM VERBOSE print these numbers, so that people finally have >some intelligent way of adjusting their FSM parameters.) > >5. When FSM space is oversubscribed, we can divide each relation's MRFS >requested page count by the oversubscription ratio to arrive at an exact >target page count for each relation. This page count is stable as long >as the space-allocation picture isn't changing much, which is a big >improvement over the existing inherently history-dependent thresholding >algorithm. > >6. A reasonably effective way to reduce a relation's stored page list >to the target (or select out the pages to actually remember from an MRFS >request) is as follows: > * Prescan the page data to compute a histogram of available-space > values, with maybe 32 bins. > * Find the histogram bin whose inclusion would make us exceed the target > page count. Set thresholdL = its lower edge value, thresholdU = its > upper edge value, and bincount = target page count minus sum of counts > in higher histogram bins. > * Scan pages. Keep all those >= thresholdU; drop those < thresholdL; > of the pages between the thresholds, keep the first bincount entries. > This approach will drop some pages with more free space while keeping > some with less --- but the difference between those dropped and those > kept is no more than the bin width. > >This algorithm still requires some heuristics around the edges --- in >particular, in step 5 we probably don't want a purely linear division of >available space, but should allow some minimum number of pages for each >relation before divvying up the leftover space. But it doesn't seem to >need nearly as much ad-hoc tuning as the thresholding approach does. > >One thing this does not do that the original code tries to do is give >preference to more-heavily-used relations --- in this approach, each >table's share of FSM is determined only by its number of pages with >free space. However, one could argue that that is an indirect measure >of activity, since it certainly reflects past deletion/update activity. >So it may be okay not to give any explicit preference to active tables. > >Comments anyone? > > regards, tom lane > >PS: Another idea I'm toying with is to dump out the FSM contents at >postmaster shutdown and reload them at restart, so that the FSM doesn't >have to start from ground zero on every restart cycle. But that's >independent of the management algorithm... > >