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Gnome shell 3.26.2 freezes when heavy swapping out happens
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Anton Gubarkov
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Joined: 13 Mar 2008
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PostPosted: Thu Jan 31, 2019 9:41 am    Post subject: Gnome shell 3.26.2 freezes when heavy swapping out happens Reply with quote

I'm running ~amd64 arch of Gentoo and I'm suffering from the following behaviour (I observe this for quite a time on all my systems running gentoo).

When there are conditions to swap out, gnome-shell is frozen until swap out is complete. I have a modern Thinkpad P52 with cofee-lake i7 processor and 32 GB of RAM. My use case is that I run several (KVM/QEMU) VMs on this host. The memory backing for these VMs are huge pages.

Sometimes when I start a VM, the memory is fragmented and there is not enough continuous memory space to allocate huge pages for the guest. The kernel reacts by swapping out fragments to free the continuous part. I can see it watching the swap utilization grow. There is still plenty of memory available at this moment (machines require 4GB each, 1 is running, another one attempts to start, no other application running except Gnome desktop).
As long as this swap out is running, I experience a complete freeze of Gnome desktop (except the mouse cursor). I can't switch windows, the system doesn't react to key presses, I can't switch VT. The CLI program running in gnome-terminal stops to produce output. Another VM SPICE window (also using huge pages - i.e. locked in memory) doesn't respond .


Can anything be done to avoid these freezes?

Thanks
Anton.
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Anton Gubarkov
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Joined: 13 Mar 2008
Posts: 39

PostPosted: Thu Jan 31, 2019 2:50 pm    Post subject: Reply with quote

I tested the same scenario with XFCE4 session - and I don't observe any freezes there. Despite heavy swapout, the desktop remains responsive.

I tried to compare the pidstat -r and I found out that gnome-shell generates 4500-5000 8O minor page faults per second. May it be the reason? XFCE4 doesn't do it.
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