Date: Wed, 14 Mar 2001 10:05:32 -0800 (PST) From: Matt Dillon <dillon@earth.backplane.com> To: Paolo Losi <p.losi@lombardiacom.it> Cc: freebsd-hackers@FreeBSD.ORG Subject: Re: systat -vmstat or iostat IO help Message-ID: <200103141805.f2EI5WJ19838@earth.backplane.com> References: <3AAF76EB.42D070F9@lombardiacom.it>
next in thread | previous in thread | raw e-mail | index | archive | help
: It will use all available memory for disk caching if possible, but it
: only caches things you've accessed at least once so I would say you
: simply haven't accessed more then 650MB or so worth of file data.
:
:Is it correct to say that cached file data (file content)
:is taken into account in Active/Inactive/Cached page counters if and only if
:accessed through the mmap interface? What does it happen when a process
:access file data through the filesystem interface (open/read/close)?
It doesn't matter how the data is accessed... either through read/write
or mmap, it will be cached the same in the system. FreeBSD does have
a sequential detection heuristic and will attempt to throw away data
earlier that it believes will not be accessed again soon, but the
heuristic is implemented both for the VM system (mmap) and for
read/write so there actually isn't much of a distinction between the
two.
:Is it correct to say that in this case the Buffer counter gives you
:the detail on how much file data is cached?
:This should be how the old 4.4BSD works....
FreeBSD has what is known as a 'unified buffer cache', as you note
below. This means that the data mapped by the buffer cache is in
fact the same data in the VM page queues, without any duplication.
The buffer cache can be thought of as a 'quick lookup' mechanism,
making it easier for the kernel to manipulate file data and also
holding file block -> disk block translations for the I/O subsystem.
If a data buffer is not in the buffer cache it may still be in the VM
page queues.
The buffer cache is typically limited to a few dozen megabytes worth of
mappings. The VM page queues are not limited in any way. So even though
the buffer counter might indicate that, say, 20MB of data is in the
buffer cache, you could still really have hundreds of megabytes of cached
file data in the VM page queues. The buffer counter does not really
represent anything.
:I know that the buffer cache management has been merged with
:the paging system in FreeBSD but I do not know if the above
:statements still hold true.
:
: Thanks
: Paolo
:
:P.S. Matt, your mentoring job is invaluable :)
Heh.
-Matt
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-hackers" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200103141805.f2EI5WJ19838>