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From: Robert Haas on 26 Jul 2010 12:33 On Mon, Jul 26, 2010 at 10:31 AM, Alvaro Herrera <alvherre(a)commandprompt.com> wrote: > Excerpts from Robert Haas's message of lun jul 26 08:52:46 -0400 2010: >> Here's another idea. �Instead of making imessages use an SLRU, how >> about having it steal pages from shared_buffers? �This would require >> segmenting messages into small enough chunks that they'd fit, but the >> nice part is that it would avoid the need to have a completely >> separate shared memory arena. �Ideally, we'd make the infrastructure >> general enough that things like SLRU could use it also; and get rid of >> or reduce in size some of the special-purpose chunks we're now >> allocating. > > What's the problem you see with "another shared memory arena"? �Right > now we allocate a single large arena, and the lot of shared_buffers, > SLRU pools, locking objects, etc are all allocated from there. �If we > want another 2 MB for "dynamic shmem", we'd just allocate 2 MB more in > that large arena and give those to this new code. But that's not a very flexible design. If you discover that you need 3MB instead of 2MB, you get to restart the entire cluster. If you discover that you need 1MB instead of 2MB, you get to either restart the entire cluster, or waste 1MB of shared memory. And since actual usage will almost certainly fluctuate, you'll almost certainly be wasting some shared memory that could otherwise be used for other purposes some of the time. Now, granted, we have this problem already today, and granted also, 2MB is not an enormous amount of memory on today's machines. If we really think that 2MB will always be adequate for every purpose for which we wish to use unicast messaging, then perhaps it's OK, but I'm not convinced that's true. It would be nice to think, for example, that this could be used as infrastructure for parallel query to stream results back from worker processes to the backend connected to the user. If you're using 16 processors to concurrently scan 16 partitions of an appendrel and stream those results back to the master, will 128kB/backend be enough memory to avoid pipeline stalls? What if there's replication going on at the same time? What if there's other concurrent activity that also uses imessages? Or even better, what if there's other concurrent activity that uses the dynamic allocator but NOT imessages? If the point of having a dynamic allocator is that it's eventually going to be used by lots of different subsystems, then we had better have a fairly high degree of confidence that it actually will, but in fact we've made very little effort to characterize who the other users might be and whether the stated implementation limitations will be adequate for them. Frankly, I doubt it. One of the major reasons why malloc() is so powerful is that you don't have to decide in advance how much memory you're going to need, as you would if you put the structure in the data segment. Dynamically allocating out of a 2MB segment gives up most of that flexibility. What I think will end up happening here is that you'll always have to size the segment used by the dynamic allocator considerably larger than the amount of memory you expect to actually be used, so that performance doesn't go into the toilet when it fills up. As Markus pointed out upthread, you'll always need some hard limit on the amount of space that imessages can use, but you can make that limit much larger if it's not reserved for a single purpose. If you use the "temporarily allocated shared buffers" method, then you could set the default limit to something like "64MB, but not more than 1/8th of shared buffers". Since the memory won't get used unless it's needed, you don't really have to care whether a particular installation is likely to need some, none, or all of that; whereas if you're allocating nailed-down memory, you're going to want a much smaller default - a couple of MB, at most. Furthermore, if you do happen to be running on a 64GB machine with 8GB of shared_buffers and 64MB isn't adequate, you can easily make it possible to bump that value up by changing a GUC and hitting reload. With the "nailed-down shared memory" approach, you're locked into whatever you decide at postmaster start. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise Postgres Company -- Sent via pgsql-hackers mailing list (pgsql-hackers(a)postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
From: Markus Wanner on 26 Jul 2010 12:51 Hi, On 07/26/2010 06:33 PM, Robert Haas wrote: > It would be nice to think, for example, that this could be used as > infrastructure for parallel query to stream results back from worker > processes to the backend connected to the user. If you're using 16 > processors to concurrently scan 16 partitions of an appendrel and > stream those results back to the master Now, *that* sounds like music to my ears ;-) Or put another way: yes, I think imessages and the bgworker infrastructure stuff could enable or at least help that goal. > Dynamically allocating out of a 2MB > segment gives up most of that flexibility. Absolutely, that's why I'd like to see other modules that use the dynamic allocator. The more the better. > What I think will end up happening here is that you'll always have to > size the segment used by the dynamic allocator considerably larger > than the amount of memory you expect to actually be used, so that > performance doesn't go into the toilet when it fills up. As Markus > pointed out upthread, you'll always need some hard limit on the amount > of space that imessages can use, but you can make that limit much > larger if it's not reserved for a single purpose. If you use the > "temporarily allocated shared buffers" method, then you could set the > default limit to something like "64MB, but not more than 1/8th of > shared buffers". I've been thinking about such rules as well. They quickly get more complex if you begin to take OOM situations and their counter-measures into account. In a way, fixing every separate pool to its specific size just is the very simples rule-set I can think of. The dynamic allocator buys you more flexibility, but choosing good limits and rules between the sub-systems is another issue. Regards Markus Wanner -- Sent via pgsql-hackers mailing list (pgsql-hackers(a)postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
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