Thread: Linux TOP
Hi, I have one of my database server that I run the "top" command: top - 16:16:30 up 42 days, 13:23, 4 users, load average: 3.13, 3.52, 3.36 Tasks: 624 total, 1 running, 623 sleeping, 0 stopped, 0 zombie Cpu(s): 1.4%us, 1.1%sy, 0.0%ni, 84.4%id, 12.9%wa, 0.0%hi, 0.2%si, 0.0%st Mem: 16432240k total, 16344596k used, 87644k free, 27548k buffers Swap: 10241428k total, 3680860k used, 6560568k free, 6230376k cached I´m afraid of two things, one is the "load average", I think 3 is too much, another is the "swap", almost 4GB of swap, I think that is too much swap. Am I right? Can I use those indicators to know if my database is ok? Thanks Waldomiro
On Wed, 21 Oct 2009, Waldomiro wrote: > I'm afraid of two things, one is the "load average", I think 3 is too > much, another is the "swap", almost 4GB of swap, I think that is too much > swap. > > Am I right? Not necessarily. > Can I use those indicators to know if my database is ok? Perhaps. Google is your friend. For example, enter the search term "linux load averages" and one of the first hits is: <http://www.lifeaftercoffee.com/2006/03/13/unix-load-averages-explained/> Do the same thing to understand swap. Rich
On Wed, 21 Oct 2009, Waldomiro wrote: > top - 16:16:30 up 42 days, 13:23, 4 users, load average: 3.13, 3.52, 3.36 > Cpu(s): 1.4%us, 1.1%sy, 0.0%ni, 84.4%id, 12.9%wa, 0.0%hi, 0.2%si, 0.0%st > Mem: 16432240k total, 16344596k used, 87644k free, 27548k buffers > Swap: 10241428k total, 3680860k used, 6560568k free, 6230376k cached > > I'm afraid of two things, one is the "load average", I think 3 is too much You're at 12.9% waiting for I/O and 84.4% idle. That means your average load consists of three processes who are stuck waiting for I/O at any given time. The I/O is what you should be worried about, not the load average. > another is the "swap", almost 4GB of swap, I think that is too much > swap. It does look like your server is using much more RAM than it actually has, which is the likely reason for all the disk I/O. If you sort the top output by memory, you might see why that is. The information provided by top on Linux isn't very good though; take a look at /proc/meminfo for more details. Rather than rely on top's math, instead I usually capture the output from: ps -e -o pid,rss,vsz,size,user,cmd And add things up myself instead, taking into account the shared bits each of the PostgreSQL processes includes. -- * Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD
On Wed, Oct 21, 2009 at 4:01 PM, Greg Smith <gsmith@gregsmith.com> wrote: > On Wed, 21 Oct 2009, Waldomiro wrote: > >> top - 16:16:30 up 42 days, 13:23, 4 users, load average: 3.13, 3.52, >> 3.36 >> Cpu(s): 1.4%us, 1.1%sy, 0.0%ni, 84.4%id, 12.9%wa, 0.0%hi, 0.2%si, >> 0.0%st >> Mem: 16432240k total, 16344596k used, 87644k free, 27548k buffers >> Swap: 10241428k total, 3680860k used, 6560568k free, 6230376k cached >> >> I'm afraid of two things, one is the "load average", I think 3 is too much > > You're at 12.9% waiting for I/O and 84.4% idle. That means your average > load consists of three processes who are stuck waiting for I/O at any given > time. The I/O is what you should be worried about, not the load average. > >> another is the "swap", almost 4GB of swap, I think that is too much swap. > > It does look like your server is using much more RAM than it actually has, > which is the likely reason for all the disk I/O. If you sort the top output > by memory, you might see why that is. This is a common misunderstanding. In this: Mem: 16432240k total, 16344596k used, 87644k free, 27548k buffers Swap: 10241428k total, 3680860k used, 6560568k free, 6230376k cached The 6.2G cached is considered part of the 16G used So it's not using more memory than it has. It's just the accounting is inobvious.
Scott Marlowe <scott.marlowe@gmail.com> writes:
> In this:
> Mem: 16432240k total, 16344596k used, 87644k free, 27548k buffers
> Swap: 10241428k total, 3680860k used, 6560568k free, 6230376k cached
> The 6.2G cached is considered part of the 16G used
> So it's not using more memory than it has. It's just the accounting
> is inobvious.
Right, but it still appears that there's something close to 14G of
actual memory use (exclusive of kernel disk buffers). If that's
the true requirement of the set of processes being run, 16G of RAM
is pretty darn marginal, and he should go buy more. But first it
would be prudent to find out where the memory is going. Also, one
thing I'd do immediately is to watch "vmstat 1" for awhile to see if
there's a lot of swap activity. If that's where the I/O is going,
it'd be another signal that memory pressure is the real issue.
regards, tom lane
On Wed, 21 Oct 2009, Scott Marlowe wrote: > In this: > > Mem: 16432240k total, 16344596k used, 87644k free, 27548k buffers > Swap: 10241428k total, 3680860k used, 6560568k free, 6230376k cached > > The 6.2G cached is considered part of the 16G used > > So it's not using more memory than it has. It's just the accounting > is inobvious. This is a snapshot. The fact that 3.7GB of swap is used here suggests there may have been more memory used at some point in the past then we're seeing now; that's more what I was commenting on. A look at the si/so figures in vmstat should nail down whether that's still going on or not now, as Tom already suggested. -- * Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD
On Wed, Oct 21, 2009 at 4:25 PM, Greg Smith <gsmith@gregsmith.com> wrote: > On Wed, 21 Oct 2009, Scott Marlowe wrote: > >> In this: >> >> Mem: 16432240k total, 16344596k used, 87644k free, 27548k buffers >> Swap: 10241428k total, 3680860k used, 6560568k free, 6230376k cached >> >> The 6.2G cached is considered part of the 16G used >> >> So it's not using more memory than it has. It's just the accounting >> is inobvious. > > This is a snapshot. The fact that 3.7GB of swap is used here suggests there > may have been more memory used at some point in the past then we're seeing > now; that's more what I was commenting on. A look at the si/so figures in > vmstat should nail down whether that's still going on or not now, as Tom > already suggested. Definitely. not arguing the guy doesn't have problems, just that the way top accounts for memory is rather misleading for most folks.