Re: PG_DUMP very slow because of STDOUT ??

Now, I have found an unorthodox way, to make a slow machine run COPY-to-STDOUT fast. I empty the cache memory of the server, which makes "free" in "free -m" jump up to 14 GBytes (well, I just noticed, that most of the memory on the server is in "cache" ... up to 22 GBytes). I just entered:

" sync;echo 3 > /proc/sys/vm/drop_caches"

Running the COPY-to-STDOUT test after this immediately went through in a snap (2 1/2 minutes). I also see, that something in relation with the file is "nicely" mapped into cache memory, because as soon as I delete the file (with "rm"), that immediately frees up 3 GBytes of the cache.

This seems to prove, that a memory issue is/was behind the slow down. But still the question remains, why and how this can happen? I mean, at some point the memory manager most have taken a very wrong decision, if this is the result of its "normal" work ... And how the writing trough the socket affects this, I don't understand (because I still see the case, when a normal COPY-to-FILE didn't slow down at the same time when COPY-to-STDOUT was crouching).

So, potentially, maybe ... as a quick fix I could clean caches in my backup script that starts each night. But is this a safe idea at all? Or could there be adverse implications (yes, of course, some queries that got their data from the file cache before would now need to repopulate it) ?

Or is there a way to influence the memory manager Linux in a way, that it behaves a bit more conservative (or just different in a positive way)?

Andras Fabian

-----Ursprüngliche Nachricht-----
Von: Craig Ringer [mailto:craig(at)postnewspapers(dot)com(dot)au]
Gesendet: Dienstag, 13. Juli 2010 12:51
An: Andras Fabian
Cc: pgsql-general(at)postgresql(dot)org
Betreff: Re: AW: AW: AW: [GENERAL] PG_DUMP very slow because of STDOUT ??

On 13/07/2010 6:26 PM, Andras Fabian wrote:
> Wait, now, here I see some correlation! Yes, it seems to be the memory! When I start my COPY-to-STDOUT experiment I had some 2000 MByte free (well ,the server has 24 GByte ... maybe other PostgreSQL processes used up the rest). Then, I could monitor via "ll -h" how the file nicely growed (obviously no congestion), and in parallel see, how "free -m" the "free" memory went down. Then, it reached a level below 192 MByte, and congestion began. Now it is going back and forth around 118-122-130 ... Obviously the STDOUT thing went out of some memory resources.
> Now I "only" who and why is running out, and how I can prevent that.

> Could there be some extremely big STDOUT buffering in play ????

Remember, "STDOUT" is misleading. The data is sent down the network
socket between the postgres backend and the client connected to that
backend. There is no actual stdio involved at all.

Imagine that the backend's stdout is redirected down the network socket
to the client, so when it sends to "stdout" it's just going to the
client. Any buffering you are interested in is in the unix or tcp/ip
socket (depending on how you're connecting), in the client, and in the
client's output to file/disk/whatever.

--
Craig Ringer