Re: [PERFORM] Sort causes system to freeze
Craig James [EMAIL PROTECTED] writes: Maybe this is an obviously dumb thing to do, but it looked reasonable to me. The problem is, the seemingly simple sort below causes a fairly powerful computer to completely freeze for 5-10 minutes. trace_sort output might be informative. regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Sort causes system to freeze
Don't reply to another message when starting a new thread. People will miss your message. Craig James wrote: Maybe this is an obviously dumb thing to do, but it looked reasonable to me. Looks reasonable here too - except I'm not sure what I'd do with 2 million rows of sorted table in my console. I'm guessing you're piping the output into something. The problem is, the seemingly simple sort below causes a fairly powerful computer to completely freeze for 5-10 minutes. During the sort, you can't login, you can't use any shell sessions you already have open, the Apache server barely works, and even if you do nice -20 top before you start the sort, the top(1) command comes to a halt while the sort is proceeding! As nearly as I can tell, the sort operation is causing a swap storm of some sort -- nothing else in my many years of UNIX/Linux experience can cause a nice -20 process to freeze. Nothing should cause that to your machine. I've never seen top just freeze unless you set up some sort of fork-bomb and ramp the load up so fast it can't cope. Oh, and nice-ing the client isn't going to do anything to the backend actually doing the sorting. The sort operation never finishes -- it's always killed by the system. Once it dies, everything returns to normal. You're running out of memory then. It'll be the out-of-memory killer (assuming you're on Linux). This is 8.3.0. (Yes, I'll upgrade soon.) Make soon more urgent than it has been up to now - no point in risking all your data to some already fixed bug is there? Unless you've been carefully tracking the release notes and have established that there's no need in your precise scenario. Is this a known bug, or do I have to rewrite this query somehow? Maybe add indexes to all four columns being sorted? Indexes won't necessarily help if you're sorting the whole table. Maybe if you had one on all four columns. = explain select * from plus order by supplier_id, compound_id, units, price; max_connections = 1000 shared_buffers = 2000MB work_mem = 256MB So can you support (1000 * 256 * 2) + 2000 MB of RAM? effective_cache_size = 4GB ...while leaving 4GB free for disk caching? Machine: Dell, 8x64-bit CPUs, 8GB ram, Perc6i battery-backed RAID controller, 8 disks as RAID10 It appears not. Remember that work_mem is not only per-connection, a single query can use multiples of it (hence the *2 above). If you genuinely have a lot of connections I'd drop it down to (say) 4MB to make sure you don't swap on a regular basis (should probably be even lower to be truly safe). Then, for the odd case when you need a large value, issue a SET work_mem before the query. -- Richard Huxton Archonet Ltd -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
[PERFORM] Sort causes system to freeze
Maybe this is an obviously dumb thing to do, but it looked reasonable to me. The problem is, the seemingly simple sort below causes a fairly powerful computer to completely freeze for 5-10 minutes. During the sort, you can't login, you can't use any shell sessions you already have open, the Apache server barely works, and even if you do nice -20 top before you start the sort, the top(1) command comes to a halt while the sort is proceeding! As nearly as I can tell, the sort operation is causing a swap storm of some sort -- nothing else in my many years of UNIX/Linux experience can cause a nice -20 process to freeze. The sort operation never finishes -- it's always killed by the system. Once it dies, everything returns to normal. This is 8.3.0. (Yes, I'll upgrade soon.) Is this a known bug, or do I have to rewrite this query somehow? Maybe add indexes to all four columns being sorted? Thanks! Craig = explain select * from plus order by supplier_id, compound_id, units, price; QUERY PLAN --- Sort (cost=5517200.48..5587870.73 rows=28268100 width=65) Sort Key: supplier_id, compound_id, units, price - Seq Scan on plus (cost=0.00..859211.00 rows=28268100 width=65) = \d plus Table emol_warehouse_1.plus Column | Type | Modifiers ---+---+--- supplier_id | integer | supplier_name | text | compound_id | text | amount| text | units | text | price | numeric(12,2) | currency | text | description | text | sku | text | Indexes: i_plus_compound_id btree (supplier_id, compound_id) i_plus_supplier_id btree (supplier_id) max_connections = 1000 shared_buffers = 2000MB work_mem = 256MB max_fsm_pages = 100 max_fsm_relations = 5000 synchronous_commit = off #wal_sync_method = fdatasync wal_buffers = 256kB checkpoint_segments = 30 effective_cache_size = 4GB Machine: Dell, 8x64-bit CPUs, 8GB ram, Perc6i battery-backed RAID controller, 8 disks as RAID10 -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
