On 22 Jan 2023, at 2:42, Mark Millard wrote:
> On Jan 22, 2023, at 00:21, Mark Millard <mark...@yahoo.com> wrote:
>
>> On Jan 21, 2023, at 23:17, Poul-Henning Kamp <p...@phk.freebsd.dk> wrote:
>>
>>> --------
>>> Mark Millard writes:
>>>
>>>> It would be nice if I could have just one swap partition
>>>> on a given boot media, one that is more than sufficient
>>>> in size for all but the biggest RAM system --but to then
>>>> be able to tell the system to just use up to the
>>>> recommended swap space size and to ignore any extra swap
>>>> space in the swap partition.
Why not just reduce the size of the swap partition to the desired size
with “gpart resize”? Granted, that requires manual intervention.
Mike
>>> Last I looked at that code, that is precisely what happens
>>> if you add a too big swap-device ?
>>
>> It produces a notice reporting how much bigger what it is
>> using is than what is recommended, if I understand the
>> message right. Here is an example were the difference was
>> small for an armv7 context:
>>
>> warning: total configured swap (1003519 pages) exceeds maximum recommended
>> amount (1003072 pages).
>>
>> Another from a context with a much bigger difference:
>>
>> warning: total configured swap (2097152 pages) exceeds maximum recommended
>> amount (916632 pages).
>>
>> These sort of messages are followed by:
>>
>> warning: increase kern.maxswzone or reduce amount of swap.
>>
>> But, as I understand, increasing kern.maxswzone makes
>> tradoffs with other kernel memory use. man 8 loader
>> reports:
>
> All my references to "man 8 loader" should have been to
> "man 8 loader_simp" these days. (Old habit, not yet
> replaced.)
>
>> kern.maxswzone
>> Limits the amount of KVM to be used to hold swap metadata,
>> which directly governs the maximum amount of swap the
>> system can support . . .
>> . . .
>> Note that swap metadata can be fragmented, which means that
>> the system can run out of space before it reaches the
>> theoretical limit. Therefore, care should be taken to not
>> configure more swap than approximately half of the
>> theoretical maximum.
>>
>> (Note: My understanding is that an "approximately half" is the
>> figure shown as the "recommended amount" in the warnings.)
>>
>> Running out of space for swap metadata can leave the system
>> in an unrecoverable state. Therefore, you should only
>> change this parameter if you need to greatly extend the KVM
>> reservation for other resources such as the buffer cache or
>> kern.ipc.nmbclusters. Modifies kernel option
>> VM_SWZONE_SIZE_MAX.
>>
>> The wording in man 8 loader is about decreasing kern.maxswzone
>
> Again.
>
>> in order to make room for other resources. But the implication
>> is that increases leave less room than normal for other
>> resources. I try to avoid getting the warnings as I do not have
>> knowledge/context to make well-guided tradeoffs for the
>> resources.
>>
>> As I understand, the 2097152 pages vs. 916632 pages example means
>> that it was operating with the referenced fragmentation problems
>> being more likely. That would not be true if it was just using
>> more like the 916632 pages and ignoring the rest.
>>
>> (I was not suggesting changes to default behavior. I was only
>> suggesting being able to put it in a mode where it would have
>> used, for example, just around 916632 pages of the swap space.)
>>
>>
>> (Note: Some of the detailed man 8 loader claims that I left out
>
> Again.
>
>> seem to not be general to all platforms, despite the wording
>> giving no hint of that issue.)
>
>
>
>
> ===
> Mark Millard
> marklmi at yahoo.com
