Date: Thu, 10 May 2012 14:33:12 -0400 From: Charles Owens <cowens@greatbaysoftware.com> To: alc@freebsd.org Cc: Adam Vande More <amvandemore@gmail.com>, Alan Cox <alan.l.cox@gmail.com>, freebsd-stable@freebsd.org Subject: Re: superpages not solving "PV entries" limit warning Message-ID: <4FAC09E8.8050309@greatbaysoftware.com> In-Reply-To: <CAJUyCcM09NsxEUz36mXvRf1w7gT5t4KG9afQwUxFZGpkndep0Q@mail.gmail.com> References: <4FAAAF8E.40007@greatbaysoftware.com> <CA%2BtpaK2nOieH%2BLFwqDXRX-DRVXvhL3%2BcXfWPoskY6-xW5Qj-_g@mail.gmail.com> <CAJUyCcO7MoT0ux9aRxNtkMo2eij=NZ%2BiQ0jsh9j2hoH6wabPhw@mail.gmail.com> <4FABE440.5000100@greatbaysoftware.com> <CAJUyCcM09NsxEUz36mXvRf1w7gT5t4KG9afQwUxFZGpkndep0Q@mail.gmail.com>
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On 5/10/12 12:37 PM, Alan Cox wrote: > On Thu, May 10, 2012 at 10:52 AM, Charles Owens > <cowens@greatbaysoftware.com <mailto:cowens@greatbaysoftware.com>> wrote: > > That's very helpful! I had read about that and wondered if it > applied to i386. > > Should I have expected "superpages" to completely cure the > condition... or does it just help? Should I now be looking at > tuning the related pmap sysctls to give further relief? > > > Superpages won't cure the problem due to the nature of your workload. > After a fork, writes to portions of the address space that are both > superpages and copy-on-write will trigger demotion, or > re-instantiation of the 4KB page granularity PV entries. Ultimately, > repromotion to superpages may occur, but in the meantime, your peak > usage of PV entries is only slightly reduced. > > The bottom line is that you'll need to resort to tuning. > > Alan Ok. Very good. Lastly, since we're talking about this (for the future) -- is enabling of superpages generally recommended for amd64? Thanks, Charles
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