[gpfsug-discuss] Memory accounting for processes writing to GPFS

[Dorigo Alvise (PSI)]

Hello to everyone,
Here a PSI we're observing something that in principle seems strange (at least to me).
We run a Java application writing into disk by mean of a standard AsynchronousFileChannel, whose I do not the details.
There are two instances of this application: one runs on a node writing on a local drive, the other one runs writing on a GPFS mounted filesystem (this node is part of the cluster, no remote-mounting).

What we do see is that in the former the application has a lower sum VIRT+RES memory and the OS shows a really big cache usage; in the latter, OS's cache is negligible while VIRT+RES is very (even too) high (with VIRT very high).

So I wonder what is the difference... Writing into a GPFS mounted filesystem, as far as I understand, implies "talking" to the local mmfsd daemon which fills up its own pagepool... and then the system will asynchronously handle these pages to be written on real pdisk. But why the Linux kernel accounts so much memory to the process itself ? And why this large amount of memory is much more VIRT than RES ?

thanks in advance,

   Alvise
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