Messages in this thread | | | Date | Tue, 14 Oct 2003 04:56:14 -0700 | From | Andrew Morton <> | Subject | Re: mem=16MB laptop testing |
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William Lee Irwin III <wli@holomorphy.com> wrote: > > So I tried mem=16m on my laptop (stinkpad T21).
Thanks for doing this. We should try to not suck in this situation.
> (a) The profile buffer requires about a 5MB bootmem allocation; > this near halves MemTotal when used. I refrained from using it, > as otherwise it's a test of mem=8m instead of mem=16m.
OK, so don't boot with `profile=N', yes?
> (b) bootmem allocations aren't adding up; after kernel text, data, > and tracing __alloc_bootmem_core(), there is still about 0.5MB > still missing from MemTotal. I still haven't found where it's > gone. mem_map's bootmem allocation also didn't show up in the > logs, but it should only be 160KB for 16MB of RAM, not 512KB. > Matt Mackall spotted this, too.
Perhaps drop a printk(size) and a dump_stack() into the bootmem allocator, then postprocess the dmesg output after it's booted?
> (c) mem= no longer bounds the highest physical address, but rather > the sum of memory in e820 entries post-sanitization. This > means a ZONE_NORMAL with about 384KB showed up, with duly > perverse heuristic consequences for page_alloc.c
I don't understand this. You mean almost all memory was in ZONE_DMA?
"mem=" does not accurately emulate having that much memory. So a 512M box booted with "mem=256M" has a different amount of memory from a 256M box booted with no "mem=" option. It would be nice to fix that, but I've never looked into it.
> (d) The system thrashed heavily on boot, allowing the largest mm > to acquire an RSS no larger than about 100KB. This needed > turning /proc/sys/vm/min_free_kb down to 128 to make the > system behave closer to normally. Matt Mackall spotted this.
hrm. min_free_kbytes is normally 1024. I'm surprised that the additional 900k made so much difference. We must be on the hairy edge.
It looks like we need to precalculate/scale min_free_kbytes, yes?
> (e) About 4.8MB are consumed by slab allocations at runtime. > The top 10 slab abusers are: > > inode_cache 840K 840K 100.00% > dentry_cache 746K 753K 99.07% > ext3_inode_cache 591K 592K 99.84% > size-4096 504K 504K 100.00% > size-512 203K 204K 99.75% > size-2048 182K 204K 89.22% > pgd 188K 188K 100.00% > task_struct 100K 108K 92.86% > vm_area_struct 93K 101K 92.28% > blkdev_requests 101K 101K 100.00% > > The inode_cache culprit is the obvious butt of many complaints: > # find /sys | wc -l > 2656 > > ... which accounts for 100% of the 840KB. TANSTAAFL. OTOH, maybe we > need to learn to do better than pinning dentries and inodes in-core...
I guess not mounting /sys doesn't help here. It would be nice. Maybe with a CONFIG_I_WILL_NEVER_MOUNT_SYSFS we could avoid all those allocations.
> Load control, anyone?
Roger Luethi is working on it; I need to pay some attention to his patch. I expect we'll have something for post-2.6.0.
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