Re: [Patch 0/7] Implement crashkernel=auto

Re: [Patch 0/7] Implement crashkernel=auto

[Eric W. Biederman]

Let me put this concrete proposal on the table.

The problem:

With the current set of crashkernel= options we are asking the
distribution installer to perform magic.  Moving as much of this logic
into a normal init script for better maintenance is desirable.

My proposal:

Implement crashkernel=max which reserves as much memory as is
reasonable for a crash kernel, without seriously affecting stability,
performance, and reliability.

As an initial approximation I would use a 32nd of low memory.

In addition implement:

/sys/kernel/crash_size

That can be written to (with enough privileges when no crash kernel is
loaded) reduce the amount of memory reserved by the crash kernel.

Bernhard does that sound useful to you?

Amerigo does that seem reasonable?

Eric

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Re: [Patch 0/7] Implement crashkernel=auto

[Andi Kleen]

> As an initial approximation I would use a 32nd of low memory.

That means a 1TB machine will have a 32GB crash kernel.

Surely that's excessive?!?

It would be repeating all the same mistakes people made with hash tables
several years ago.

> 
> That can be written to (with enough privileges when no crash kernel is
> loaded) reduce the amount of memory reserved by the crash kernel.
> 
> Bernhard does that sound useful to you?
> 
> Amerigo does that seem reasonable?

It doesn't sound reasonable to Andi.

Why do you even want to grow the crash kernel that much? Is there
any real problem with a 64-128MB crash kernel?

-Andi
> 

-- 
ak@linux.intel.com -- Speaking for myself only.

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Re: [Patch 0/7] Implement crashkernel=auto

[Bernhard Walle]

Andi Kleen schrieb:
>> As an initial approximation I would use a 32nd of low memory.
> 
> That means a 1TB machine will have a 32GB crash kernel.
> 
> Surely that's excessive?!?
> 
> It would be repeating all the same mistakes people made with hash tables
> several years ago.

The idea of Eric was to shrink the reserved memory in an init script. I
doubt that the 1 TB machine will have any problems or performance issue
when booting with (1 TB - 32 GB) memory.

> It doesn't sound reasonable to Andi.
> 
> Why do you even want to grow the crash kernel that much? Is there
> any real problem with a 64-128MB crash kernel?

Try it out. No chance for 64-128MB crashkernel on "medium" IA64 machines.


Regards,
Bernhard

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Re: [Patch 0/7] Implement crashkernel=auto

[Bernhard Walle]

Eric W. Biederman schrieb:
> 
> With the current set of crashkernel= options we are asking the
> distribution installer to perform magic.  Moving as much of this logic
> into a normal init script for better maintenance is desirable.

Not (necessarily) the installer but the program that configures kdump.
system-config-kdump on Red Hat, YaST on SUSE.

> Bernhard does that sound useful to you?

I don't see any problems. I don't know how much effort is it to free
already reserved crashkernel memory, but I guess it's not really
complicated. Maybe that "1/32" should be specified on the command line like


	crashkernel=>>5

(for 1/32*system_memory == system_memory>>5), OTOH I have no real strong
opinion.


Regards,
Bernhard

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Re: [Patch 0/7] Implement crashkernel=auto

[Eric W. Biederman]

Andi Kleen <andi@firstfloor.org> writes:

>> As an initial approximation I would use a 32nd of low memory.
>
> That means a 1TB machine will have a 32GB crash kernel.
>
> Surely that's excessive?!?
>
> It would be repeating all the same mistakes people made with hash tables
> several years ago.
>
>> 
>> That can be written to (with enough privileges when no crash kernel is
>> loaded) reduce the amount of memory reserved by the crash kernel.
>> 
>> Bernhard does that sound useful to you?
>> 
>> Amerigo does that seem reasonable?
>
> It doesn't sound reasonable to Andi.
>
> Why do you even want to grow the crash kernel that much? Is there
> any real problem with a 64-128MB crash kernel?

Because it is absolutely ridiculous in size and user space will have
to take up the work of trimming back down to something reasonable in
the init script.

At a practical level crash dump userlands do things like fsck
filesystems before they mount them.  For truly large machines there
was a desire to parallelize core dump writing to different disks.  I
don't know if that has been implemented yet, but in that case you
certainly more ram for buffers tends to be useful.

I think if we are going to go beyond having a magic boot command
line (that we have today) that parametrizes the amount of memory
to reserve based on how much memory we have in the system.  We need
to put user space in control.  We can only put user space in control
if we initially reserve too much and let it release the memory it
won't use.

That would allow removing magic from installers and leaving it to 
installed packages.  Which seems a lot more maintainable. 

Eric

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Re: [Patch 0/7] Implement crashkernel=auto

[Eric W. Biederman]

Bernhard Walle <bernhard.walle@gmx.de> writes:

> Eric W. Biederman schrieb:
>> 
>> With the current set of crashkernel= options we are asking the
>> distribution installer to perform magic.  Moving as much of this logic
>> into a normal init script for better maintenance is desirable.
>
> Not (necessarily) the installer but the program that configures kdump.
> system-config-kdump on Red Hat, YaST on SUSE.

Right.  Somehow I thought YaST was the installer my mistake.

>> Bernhard does that sound useful to you?
>
> I don't see any problems. I don't know how much effort is it to free
> already reserved crashkernel memory, but I guess it's not really
> complicated. 

Right.

> Maybe that "1/32" should be specified on the command line like
>
>
> 	crashkernel=>>5
>
> (for 1/32*system_memory == system_memory>>5), OTOH I have no real strong
> opinion.

The idea is for the system to give us as much as it can stand and
userspace gives the rest back.  The maximum memory any particular
kernel can stand to give up is a tractable kernel level problem, and
we can make it autotune like any other kernel tunable.  What a crash
kernel needs totally depends on the implementation.

Eric

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Re: [Patch 0/7] Implement crashkernel=auto

[Amerigo Wang]

Eric W. Biederman wrote:
> Let me put this concrete proposal on the table.
>
> The problem:
>
> With the current set of crashkernel= options we are asking the
> distribution installer to perform magic.  Moving as much of this logic
> into a normal init script for better maintenance is desirable.
>
> My proposal:
>
> Implement crashkernel=max which reserves as much memory as is
> reasonable for a crash kernel, without seriously affecting stability,
> performance, and reliability.
>   

This is almost exactly what I want with crashkernel=auto....
So there's no big difference, except the name.

> As an initial approximation I would use a 32nd of low memory.
>   

Hmm, I think Bernhard's proposal is fine for this case, i.e. we can 
introduce a new syntax, "crashkernel=>>X" which means we reserve 1/2^X 
of system memory.

What do you think?

> In addition implement:
>
> /sys/kernel/crash_size
>
> That can be written to (with enough privileges when no crash kernel is
> loaded) reduce the amount of memory reserved by the crash kernel.
>   

Yeah, this is nice!

Thanks.


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