Re: To Swap, or not to Swap

Respectfully, and not to be argumentative, we don't know that to be true.

Guidance from Red Hat (HERE
<https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/managing_storage_devices/getting-started-with-swap_managing-storage-devices>) in section 14.2 shows that swapfile size is proportional to the amount of physical memory, but doesn't explain precisely how.

So while a larger swapfile can be inferred to be more useful with very large volumes of RAM, even the smallest swapfile might yet provide salvation in situations.

On 3/30/2023 10:56 PM, Jared Still wrote:
> I would respectfully mention that a 16G swap partition is unlikely to
> be the salvation of a server with 1T of RAM.  :)
>
> On Thu, Mar 30, 2023 at 19:35 Clay Jackson (cjackson)
> <Clay.Jackson_at_quest.com> wrote:
>
> Yes, thanks for the exercise!
>
> Tim - very good points, as always.    Based on all of this, I
> would assert that the answer (to the question “To Swap, or not to
> Swap”) is really, “It depends”.
>
> Tim makes the point that “fail fast and recover” is better than
> “Fail slow and recover”. I can definitely agree with that.
>
> But what if we had an option, “Gracefully degrade, notify and
> recover”?   I would think degradation, for a short period of time,
> is better than a failure.  It seems that Tim’s fast storage, along
> with monitoring of swap usage, would allow for that.
>
> My father (an Episcopal, Anglican for those not in the US, priest)
> used to say, “In the end, all ethics are situational”.  If I can
> paraphrase – “In the end, the “correct” solution to a (potential)
> failure always depends on the situation”.
>
>
> Clay Jackson
>
> *From:* oracle-l-bounce_at_freelists.org
> <oracle-l-bounce_at_freelists.org> *On Behalf Of *Tim Gorman
> *Sent:* Thursday, March 30, 2023 5:25 PM
> *To:* jkstill_at_gmail.com; Oracle-L Freelists <oracle-l_at_freelists.org>
> *Subject:* Re: To Swap, or not to Swap
>
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>
> Jared,
>
>
>
> You've made a good point with your testing.  In essence, /fail
> fast/.  If it is just /fail fast/ versus /fail slow/, then of
> course we all choose to /fail fast/ and then recover.
>
> The only question that comes to my mind is whether the presence of
> a swapfile always means slow failure.
>
> Are there no longer any scenarios where the swapfile allows the
> system to recover, without failing or hanging?
>
> For example, in Azure, VMs can use remote storage (a.k.a. OsDisk)
> for the swapfile, or VMs can locate the swapfile on optional
> direct-attached SSD storage that is considered "temporary" or
> ephemeral, because when the VM is stopped and deallocated, the
> direct-attached storage has to be erased, because another VM may
> be allocated to it in future.  It is not quality of storage that
> makes it "ephemeral", just the use-case.  Anyway, the OsDisk has
> I/O latency averaging 0.70 ms for both reads and writes, but the
> so-called "ephemeral" disk provides less than 0.05 ms I/O latency,
> which is about 14x faster.
>
> Clearly the performance of the storage on which the swapfile
> resides is going to make a difference in its usefulness.  If your
> testing involved slow storage, then I can see where the machine
> would take 7-8 mins to fail.  I'm not trying to denigrate the
> resources you used, but I'm trying to ask if the swapfile is on
> fast storage, then perhaps could it be more helpful, even in
> extreme situations?
>
> In other words, shouldn't we ensure that a swapfile is fast, as
> well as big enough?  Wouldn't more performant storage allow the
> swapfile to recover the situation?
>
> Thanks so much for the thought exercise!
>
> -Tim
>
> On 3/30/2023 10:46 AM, Jared Still wrote:
>
> I was recently asked by a colleague this same question.
>
> He had been asked by a client, with a fairly well regarded
> sysadmin team.
>
> They wanted to eliminate swap: here's why.
>
> If a process is consuming memory at a prodigious rate, then
> the OOM (out of memory) killer is going to catch up to it and
> kill it eventually.
>
> Their position was that with a swap partition, this process
> was prolonged far too long.
>
> Without swap, the process gets killed relatively quickly.
>
> With swap, it can take many minutes. The CPU spends so much
> time managing memory on swap (remember, we are at an OOM
> condition), which is slow, that the time to kill the process
> is prolonged to many minutes.
>
> At first my position was "what, no swap! we can't do that!"
>
> But, I decided to test it a bit.
>
> A small physical server, 4 cores and 32G of RAM, is running
> Oracle 19.3.
>
> A swingbench test is running, 10 sessions per core.
>
> When I cause an OOM condition with the 16G swap partition
> enabled, it took the system between 7.5-8 minutes to kill the
> process.
>
> (For the client, the amount of time was 20+ minutes.)
>
> And during that time, it was impossible to logon to the
> server.  The CPU was too busy thrashing around in the swap
> partition.
>
> The next step of course is to disable the swap.
>
> Same OOM condition caused. Time to resolution is now 7 seconds.
>
> There is no swap to manage as if it were RAM.
>
> That is quite a bit difference.
>
> Of course I wondered 'what about paging in memory for new
> processes?', as that often uses a page in swap.
>
> Without swap, it just takes place in memory.
>
> Swap is also a landing place for some pages used to initialize
> processes, as they can only be used once.
>
> This is a minimal amount, and can just be left in memory.
>
> If one really wants to conserve, there is a thing called ZRAM
> (compressed memory) where those pages can be parked, instead
> of swap.
>
> So, does anyone see any other need for a swap partition?
>
> It seems to have outlived its usefulness.
>
> Jared Still
> Certifiable Oracle DBA and Part Time Perl Evangelist
>
> Principal Consultant at Pythian
>
> Oracle ACE Alumni
>
> Pythian Blog http://www.pythian.com/blog/author/still/
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> On Thu, Mar 30, 2023 at 9:24 AM Jared Still
> <jkstill_at_gmail.com> wrote:
>
> That is the question.
>
> I am curious about current thoughts on having or not
> having a swap partition on Linux based Oracle servers.
>
> Let's assume typical production standard servers with a
> reasonable amount of RAM, sway 256G or more.
>
> I  have some thoughts on this myself, but would like to
> see others' thoughts on this.
>
> Jared Still
> Certifiable Oracle DBA and Part Time Perl Evangelist
>
> Principal Consultant at Pythian
>
> Oracle ACE Alumni
>
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> --
> Jared Still
> Certifiable Oracle DBA and Part Time Perl Evangelist
> Principal Consultant at Pythian
> Oracle ACE Alumni
> Pythian Blog http://www.pythian.com/blog/author/still/
> Github: https://github.com/jkstill
> Personality: http://www.personalitypage.com/INTJ.html
>
>

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