On May 14, 2009, at 10:24 PM, prakash sangappa wrote:
However, a new set of interfaces called VPM was introduced
which provide transient mappings to file pages similar, to segmap. The
VPM interfaces use KPM mappings. The UFS, NFS, TMPFS and
SPECfs filesystem use the VPM interface where available.
Segmap is still available and is in limited use, there should be no
need
to tweak the segmap size when VPM is available.
I see. Thank you very much for this information.
If I may, I'd like to give some background on why I ask about VPM and
how the segmap now relates to it so I can understand it better vs. the
old page cache mechanism and its accounting.
I have a bunch of NFSv3 clients that have a read-only mount back to a
NFS server. While they all have the same share mounted (read only),
they each access different files on that share.
Prior to VPM-enabled kernels, I set the segmapsize on each client to
8GB (each NFS client has 16GB of RAM.) I would monitor the segmap
consumption through the ::memstat dcmd in mdb, inspecting the Page
Cache category. On pre-VPM kernels, the Page Cache would get very
reasonably close to that defined segmapsize limit of 8GB. The Free
(cachelist) tended to consume the balance.
Now with VPM on the scene, things seem unexpectedly different, obvious
to me know through your explanation. The Page Cache category reported
by ::memstat on each of the aforementioned VPM-enabled NFS clients now
seems to top out, consistently across these clients, at 12% or 1950MB
of physmem (16GB). The Free (cachelist) category does not grow as fast
as I expect it to, with several GB of RAM still on the freelist.
The 12% number strikes me as particularly odd since that was/is the
default size of the segmap on *SPARC* (segmap_percent)... so I'm a bit
perplexed as these are x64 clients we're talking about. Is that by
design or just a crazy coincidence. My trollings through the source
code doesn't reveal such a defined limit on x64. Every client is like
this.
My primary goal is to keep as many pages from the numerous but large
files these NFS clients access cached as much as possible... this is
why I set the segmapsize much higher than its 64MB default. I guess my
problem (or perhaps misunderstanding) is that this doesn't appear to
be happening anymore on these VPM-enabled kernel, regardless of what
segmapsize is set to. Perhaps I just don't understand how to find and
account for pages cached via VPM?
Thanks again
/dale
So, if you are concerned about the excessive cross calls issues
associated with the traditional segmap mappings, that has been
addressed by the VPM interfaces.
The VPM interface is also available for x64 systems on S10
starting update 6(I believe).
-Prakash.
Thanks for any insight
/dale
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