LTTng » LTTng-tools

• bugs@lttng.org (bugs@lttng.org) wrote:

Issue #228 has been reported by Matthew Khouzam.

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Bug #228: Subbuffer sizes and powers of 2
https://bugs.lttng.org/issues/228

Author: Matthew Khouzam
Status: New
Priority: Normal
Assignee:
Category:
Target version:

A subbuffer should be able to take any value, it is unreasonable to ask a user in userland to memorize the powers of 2 greater than 16.

A good compromise would be to cause lttng tool to create subbuffers that are rounded up to the nearest power of 2. I can personnaly garanty that a user asking for 1000000 bytes per subbuffer will not be outraged if the buffer is in reality 1024^2.

Another useful thing would be to have human friendly input like 4k 16k 1m 2g...

Agreed, this should be done in lttng cmd line tool. Patch is welcome,

Thanks,

Mathieu

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Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com

Please implement by pulling lttng-ust libringbuffer/backend_internal.h get_count_order() into lttng-tools. This code is already tested, and FWIW more efficient than the patch you proposed. Moreover, it follows the project coding style.

Thanks,

Mathieu

That's a pretty cool function. What is a FLS? I can tell the lttng_ust_fls() function is more desirable for this than get_count_order since if we give ti a subbuffer of 2^n, we don't need to allocate 2^n+1... or do we?

Also, the only comments are "arch-agnostic implementation" event though that is great to know, I think we really need to know what is a fls even more and what get_count_order does.

• bugs@lttng.org (bugs@lttng.org) wrote:

Issue #228 has been updated by Matthew Khouzam.

That's a pretty cool function. What is a FLS? I can tell the lttng_ust_fls() function is more desirable for this than get_count_order since if we give ti a subbuffer of 2^n, we don't need to allocate 2^n+1... or do we?

Also, the only comments are "arch-agnostic implementation" event though that is great to know, I think we really need to know what is a fls even more and what get_count_order does.

fls = find last bit set. There is a similar implementation in the Linux
kernel, so you can find its documentation there.

fls has corner-cases with the exact power values: it's not the semantic
we need. We want get_count_order. get_count_order, if given 2^n, returns
2^n, not 2^(n+1) like you assume.

Thanks,

Mathieu

----------------------------------------
Bug #228: Subbuffer sizes and powers of 2
https://bugs.lttng.org/issues/228#change-775

Author: Matthew Khouzam
Status: New
Priority: Normal
Assignee:
Category:
Target version:

A subbuffer should be able to take any value, it is unreasonable to ask a user in userland to memorize the powers of 2 greater than 16.

A good compromise would be to cause lttng tool to create subbuffers that are rounded up to the nearest power of 2. I can personnaly garanty that a user asking for 1000000 bytes per subbuffer will not be outraged if the buffer is in reality 1024^2.

Another useful thing would be to have human friendly input like 4k 16k 1m 2g...

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Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com