Re: To Swap, or not to Swap

Swap functions as a buffer for memory management, providing flexibility and resilience to a system, much like a central bank's role in stabilizing an economy. The concept of an operating system (OS) arbitrarily terminating processes under memory pressure raises concerns, particularly when it could disrupt critical operations nearing completion, such as a lengthy index build with only a few minutes remaining.

Even without swap, systems can become unstable and challenging to use. Moreover, swap usage serves as an early warning signal for memory-related issues. When paging activity is detected, engineers can investigate, diagnose, and make informed decisions about which processes to terminate, potentially through automated decision-making mechanisms.

Swap has been an essential component for memory management, providing flexibility and resilience to systems. However, in today's environments swap is often disabled. This change raises the question: How can we effectively detect memory issues without swap?

In the past, paging activity on performance monitoring dashboards was a clear indicator of memory pressure, and even minimal activity warranted further investigation. However, finding a suitable solution in modern infrastructure has proven to be challenging.

MemoryAvailable is a metric some engineers use, but its reliability has been debated due to the way UNIX-based systems manage memory. It's not always clear where to draw the line with MemoryAvailable, and determining a formula for identifying a problem remains uncertain.

One potential approach could be using memory scan rate metrics to track memory pressure in the absence of swap, but there might be other, more effective solutions. If anyone has suggestions or experiences with monitoring memory in these situations, I would love to hear your ideas. How do you detect memory issues when swap is disabled, and what metrics have you found most reliable?

Kyle

On Fri, Mar 31, 2023 at 9:12 AM Tim Gorman <tim.evdbt_at_gmail.com> wrote:

> 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
>>
>>
>>
>> *CAUTION:* This email originated from outside of the organization. Do
>> not follow guidance, click links, or open attachments unless you recognize
>> the sender and know the content is safe.
>>
>>
>>
>> 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/
>> <https://nam12.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.pythian.com%2Fblog%2Fauthor%2Fstill%2F&data=05%7C01%7Cclay.jackson%40quest.com%7Cfbf6cd8a6a334c5d07f208db317e87dc%7C91c369b51c9e439c989c1867ec606603%7C0%7C0%7C638158191761986020%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=Vd3C8P%2FF50M0DeRjlz2Ci2B7RMG7qV8bZxXo4ZwO0IU%3D&reserved=0>
>>
>> Github: https://github.com/jkstill
>> <https://nam12.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fjkstill&data=05%7C01%7Cclay.jackson%40quest.com%7Cfbf6cd8a6a334c5d07f208db317e87dc%7C91c369b51c9e439c989c1867ec606603%7C0%7C0%7C638158191761986020%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=o8p6oyzzhW6jT8ruSatnHahpXqmVRY5YxHEsiUV%2ByoE%3D&reserved=0>
>>
>> Personality: http://www.personalitypage.com/INTJ.html
>> <https://nam12.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.personalitypage.com%2FINTJ.html&data=05%7C01%7Cclay.jackson%40quest.com%7Cfbf6cd8a6a334c5d07f208db317e87dc%7C91c369b51c9e439c989c1867ec606603%7C0%7C0%7C638158191762142238%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=XdWV2p8U%2FQSoUfmAGNLoVVmEBBJei0j4Ju4JmB9VJ8Q%3D&reserved=0>
>>
>>
>>
>>
>>
>> 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
>>
>> Pythian Blog http://www.pythian.com/blog/author/still/
>> <https://nam12.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.pythian.com%2Fblog%2Fauthor%2Fstill%2F&data=05%7C01%7Cclay.jackson%40quest.com%7Cfbf6cd8a6a334c5d07f208db317e87dc%7C91c369b51c9e439c989c1867ec606603%7C0%7C0%7C638158191762142238%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=J%2BcP9lUHkE1euYUvwsXE1YCIpA%2BetWLtOos3gBhEdFo%3D&reserved=0>
>>
>> Github: https://github.com/jkstill
>> <https://nam12.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fjkstill&data=05%7C01%7Cclay.jackson%40quest.com%7Cfbf6cd8a6a334c5d07f208db317e87dc%7C91c369b51c9e439c989c1867ec606603%7C0%7C0%7C638158191762142238%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=FFYcaad4HjRIFbUlwYdQATouwGbF3BAmBKNgYD6f8P8%3D&reserved=0>
>>
>> Personality: http://www.personalitypage.com/INTJ.html
>> <https://nam12.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.personalitypage.com%2FINTJ.html&data=05%7C01%7Cclay.jackson%40quest.com%7Cfbf6cd8a6a334c5d07f208db317e87dc%7C91c369b51c9e439c989c1867ec606603%7C0%7C0%7C638158191762142238%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=XdWV2p8U%2FQSoUfmAGNLoVVmEBBJei0j4Ju4JmB9VJ8Q%3D&reserved=0>
>>
>>
>>
> --
> 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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