Date: Thu, 9 Mar 2017 02:13:40 +0300 From: Slawa Olhovchenkov <slw@zxy.spb.ru> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-hackers@freebsd.org, Karl Denninger <karl@denninger.net> Subject: Re: Kernel UMA current occupancy statistics Message-ID: <20170308231340.GT15630@zxy.spb.ru> In-Reply-To: <2800890.fCNZHq2Y8P@ralph.baldwin.cx> References: <18fdca3b-9002-db60-504b-388c628fab0e@denninger.net> <2800890.fCNZHq2Y8P@ralph.baldwin.cx>
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On Wed, Mar 08, 2017 at 02:27:15PM -0800, John Baldwin wrote:
> On Wednesday, August 31, 2016 08:02:06 AM Karl Denninger wrote:
> > Working on the ZFS ARC code I'm trying to find documentation on the
> > means by which I can determine the occupancy of a UMA slab.
> >
> > There is a userland set of calls but I assume those are not the correct
> > way to approach this in the kernel context. include/sys/vm/uma.h
> > declares that the returned structure is to be opaque to users of the
> > facility, and the only occupancy-related function I can find is
> > uma_zone_get_cur, which gives me the number of items allocated but
> > uma_zone_get_max states that it will return "0" if no limit on
> > allocations has been set.
> >
> > Any hints on how to determine, if for example there are 50,000 "units"
> > of memory that are currently held out of kmem in a given slab how many
> > are actually allocated and how many are free and reusable without a
> > further kernel memory allocation?
> >
> > What I'm trying to determine is this (from vmstat -z):
> >
> > ITEM SIZE LIMIT USED FREE REQ FAIL SLEEP
> > zio_buf_512: 512, 0, 79865, 199359, 6495950, 0, 0
> >
> > In other words how do I programmatically, inside a kernel routine (in
> > this case zfs/arc.c) retrieve the "used" and "free" values if I have a
> > given slab's pointer (which I can use to call kmem_cache_reap_now)?
> >
> > Thanks in advance;
>
> vmstat -z retrieves these using memstat_sysctl_uma() from libmemstat
> which in turn uses the vm.zone_stats sysctl. Looking in that function,
> it seems that the used field (mt_count) is computed this way:
>
> mtp->mt_count = mtp->mt_numallocs - mtp->mt_numfrees;
>
> Where numallocs and numfrees are computed by summing a set of per-CPU
> stats:
>
> for (j = 0; j < maxcpus; j++) {
> mtp->mt_numallocs += upsp->ups_allocs;
> mtp->mt_numfrees += upsp->ups_frees;
> }
>
> The free field (mt_free) is calculated similarly. First with a global
> count followed by per-CPU counts:
>
> mtp->mt_numfrees = uthp->uth_frees;
> for (j = 0; j < maxcpus; j++) {
> mtp->mt_free += upsp->ups_cache_free;
> }
>
> The vm.zone_stats sysctl is implemented by sysctl_vm_zone_stats() in
> sys/vm/uma_core.c.
>
> It seems like 'uma_zone_get_cur()' might give you USED. You would
> need to add a new function to let you calculate FREE. You can probably
> use 'uma_zone_get_cur()' as a template for implementing that other
> function. uma_zone_sumstat() might also be useful as a reference.
I am wrote such function as part of D7538:
https://reviews.freebsd.org/D7538#f080cd65
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