Bug 2041537 - lto1-* helpers eat a lot of CPU
Summary: lto1-* helpers eat a lot of CPU
Keywords:
Status: CLOSED EOL
Alias: None
Product: Fedora
Classification: Fedora
Component: gcc
Version: 35
Hardware: Unspecified
OS: Unspecified
unspecified
unspecified
Target Milestone: ---
Assignee: Jakub Jelinek
QA Contact: Fedora Extras Quality Assurance
URL:
Whiteboard:
Depends On:
Blocks:
TreeView+ depends on / blocked
 
Reported: 2022-01-17 16:11 UTC by Vít Ondruch
Modified: 2025-07-07 19:17 UTC (History)
11 users (show)

Fixed In Version:
Clone Of:
Environment:
Last Closed: 2022-12-13 16:20:52 UTC
Type: Bug
Embargoed:


Attachments (Terms of Use)

Description Vít Ondruch 2022-01-17 16:11:17 UTC
Description of problem:
When being on meeting and compiling some C code with LTO enable, the meeting is crackling and skipping and what not. It seems that this is due to various LTO helpers, such as lto1-ltrans runs. So would it be possible to somehow reduce priority of these, so the computer is still usable for foreground tasks?


Version-Release number of selected component (if applicable):
gcc-11.2.1-7.fc36.x86_64


How reproducible:


Steps to Reproduce:
1.
2.
3.

Actual results:
Build on backtround the GUI kind of sluggish, especially meetings in a browser


Expected results:
Gui and meetings are smooth with build on background.


Additional info:

Comment 1 Dave Malcolm 2022-01-17 16:32:54 UTC
LTO is going to be compute-intensive.  It sounds like you're invoking gcc on a desktop machine that you're also doing teleconferencing on (perhaps a laptop), and the compute-intensive compiler tasks are in the same control group as the interactive tasks.

This sounds like the sort of resource-control issue that's more the domain of e.g. systemd, or the desktop; it feels kind of out-of-scope for gcc to be managing such things (rather, gcc expects to *be* managed).  If e.g. you're invoking the build from a terminal on a laptop, does the terminal have a way of e.g. putting the tab's processes in a control group, and letting you tweak the priority/resource limits of that control group?  (and potentially even pausing that tab's processes)

I suppose gcc could potentially create a control group when it launches LTO, to hold all of the subprocesses, but it does feel to be out-of-scope; surely it makes more sense to have the build resource-constrained/prioritized? (rather than singling out LTO?)

Hope this is constructive.

Comment 2 Vít Ondruch 2022-01-18 13:18:37 UTC
(In reply to Dave Malcolm from comment #1)
> LTO is going to be compute-intensive.  It sounds like you're invoking gcc on
> a desktop machine that you're also doing teleconferencing on (perhaps a
> laptop),

That is correct

> and the compute-intensive compiler tasks are in the same control
> group as the interactive tasks.

Possibly, although mock and therefore systemd-nspawn might be in different control group.

> This sounds like the sort of resource-control issue that's more the domain
> of e.g. systemd, or the desktop; it feels kind of out-of-scope for gcc to be
> managing such things (rather, gcc expects to *be* managed).

I don't agree. For example, wile the number of compilation threads can be adjusted by `make -j`, the number of executed lto workers seems to be arbitrary (at least from my perspective, not long ago, I was completely unaware of them). Also, these helpers might run with lower priority, why not?

BTW, during the meeting, I tried to send the mock build into the background via Ctrl+Z, but the workers kept running.

>  If e.g. you're
> invoking the build from a terminal on a laptop, does the terminal have a way
> of e.g. putting the tab's processes in a control group, and letting you
> tweak the priority/resource limits of that control group?  (and potentially
> even pausing that tab's processes)

While gnome-terminal likely executes every tab in its control-group, I don't think it should be of my concern. I think that gcc and whatever helpers it needs should always run with background priority in case of developer machine, simple because the compilation of any bigger project takes some time no matter how powerful the machine is. At least prior enabling the LTO, it was never the issue and that is why I am here.

Comment 3 Ben Cotton 2022-11-29 17:40:44 UTC
This message is a reminder that Fedora Linux 35 is nearing its end of life.
Fedora will stop maintaining and issuing updates for Fedora Linux 35 on 2022-12-13.
It is Fedora's policy to close all bug reports from releases that are no longer
maintained. At that time this bug will be closed as EOL if it remains open with a
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Comment 4 Ben Cotton 2022-12-13 16:20:52 UTC
Fedora Linux 35 entered end-of-life (EOL) status on 2022-12-13.

Fedora Linux 35 is no longer maintained, which means that it
will not receive any further security or bug fix updates. As a result we
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