tech-kern: VM perf evaluation on i386

Subject: VM perf evaluation on i386
To: 'tech-kern@netbsd.org' <tech-kern@netbsd.org>
From: Ming Lei <mlei@brocade.com>
List: tech-kern
Date: 10/28/2003 15:37:16
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Does anyone have any opinion on the VM performance on linux vs. netbsd(UVM)
on i386. From my understand of both kernels, linux uses kind of one-to-one
mapping with a giant TLB mapping for most of its kernel space except vmalloc
and netbsd uses pure VM approach(VM and PM is different). 

Look at linux pci_alloc_consistent() just calls the physical mem allocator
and what physical allocator does is just remove the number of pages from
zone freelist; on the other hand, netbsd uses bus_dmamem_alloc() which in
turns calls uvm_pglistalloc() to loop through all the free pages to see
which is first fit for the allocate size; and then bus_dmamem_map() to map
PM with VM which is allocated by uvm_map(also does kind of transverse on map
entries).

So what I think here is in order to get flexibility of VM, netbsd sacrifices
some extent of performace(also one can count the availablity of TLB entries
for user space; netbsd kernel may use multiple TLB entries VS. linux uses
only one). I am mostly interested in seeing some real benchmark on VM perf.
Dont know if there is one.

Ming

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<TITLE>VM perf evaluation on i386</TITLE>
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<P><FONT SIZE=3D2>Does anyone have any opinion on the VM performance on =
linux vs. netbsd(UVM) on i386. From my understand of both kernels, =
linux uses kind of one-to-one mapping with a giant TLB mapping for most =
of its kernel space except vmalloc and netbsd uses pure VM approach(VM =
and PM is different). </FONT></P>

<P><FONT SIZE=3D2>Look at linux pci_alloc_consistent() just calls the =
physical mem allocator and what physical allocator does is just remove =
the number of pages from zone freelist; on the other hand, netbsd uses =
bus_dmamem_alloc() which in turns calls uvm_pglistalloc() to loop =
through all the free pages to see which is first fit for the allocate =
size; and then bus_dmamem_map() to map PM with VM which is allocated by =
uvm_map(also does kind of transverse on map entries).</FONT></P>

<P><FONT SIZE=3D2>So what I think here is in order to get flexibility =
of VM, netbsd sacrifices some extent of performace(also one can count =
the availablity of TLB entries for user space; netbsd kernel may use =
multiple TLB entries VS. linux uses only one). I am mostly interested =
in seeing some real benchmark on VM perf. Dont know if there is =
one.</FONT></P>

<P><FONT SIZE=3D2>Ming</FONT>
</P>

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